Circle Area Chart on Visualizing.JP

A Country's Extent as the Area of a Circle, the Weight of Its Tax as the Direction of a Dotted Line: Playfair's The Statistical Breviary, Chart 1

Sun, 12 Oct 2025 00:00:00 +0900

In 1801, in Chart 1 of The Statistical Breviary, William Playfair (1759–1823) drew the extent of the countries of Europe as the area of circles. Population and revenue are shown by vertical lines standing on either side of each circle, measured on the same scale. The tops of the two vertical lines are joined by a dotted line. In his text, Playfair writes that which way the dotted line rises shows whether taxes are heavy in proportion to population.

The sources are the book’s preface, pp.13–16, where he explains how to read the chart, and the country-by-country tables. I read the text in the Bodleian Library copy on the Internet Archive. Because the foldout plates were not captured correctly in that scan, I show the chart from the 1802 French translation. How the circles are divided is covered in an article comparing the two circles of 1801 and 1805, and the 1805 chart is covered in an article on the 1805 United States pie chart.

Photograph of the plate of Chart 1 in The Statistical Breviary

Chart 1 in the French translation. Circles are arranged in order of size, from the large Russia at the far left to the United States at the far right. Red and yellow vertical lines stand on either side of each circle, with their tops joined by a dotted line

The whole plate, original drawing by William Playfair, engraved by Adam (French translation, 1802). Source: David Rumsey Map Collection, David Rumsey Map Center, Stanford Libraries / Internet Archive (CC BY-NC-SA 3.0)

Four Charts Born from the Supplement to a Table

According to the preface, it began with helping with someone else’s book. The editor of the English edition of Boetticher’s statistical tables asked him for a table supplying the changes since publication. Partway through that work, he writes, he realized the following (preface p.3).

it occurred to me, that tables are by no means a good form for conveying such information

In short: tables are not at all a good form for conveying this kind of information. On p.4 he says that when proportion and magnitude are at issue, appealing to the eye is the best and quickest way.

The book appeared in London in 1801. According to Chris Pritchard, it runs to 64 pages, most of them tables by country, with four plates. The contents of the four are listed on p.14.

Chart 1: Europe before the French Revolution
Chart 2: Europe as projected by the peace of Lunéville
Chart 3: The populations of the capitals of Europe
Chart 4: The countries of Hindustan

The original wording for Chart 1 is “Exhibits the powers of Europe as they were previous to the French Revolution.” What is drawn is Europe before the Revolution, not Europe as of 1801. 1801 was the middle of the French Revolutionary Wars, and Chart 2 draws the picture after the Treaty of Lunéville in February (Pritchard 2021).

Charts built from lines were not new for Playfair. On p.15 he writes that about fifteen years earlier he had applied lines to commerce and finance; this refers to The Commercial and Political Atlas. The Statistical Breviary came out in Paris the following year, 1802, in a French translation by D.-F. Donnant, and the plates were re-engraved with French text.

The Reading Instructions Are in Two Pages of the Text

Chart 1 has no legend. How to read it is set out in the text, pp.13–14. The key passage about the vertical lines is this.

The yellow lines on the right of each nation represent the revenue in millions of pounds sterling, measured also upon the same divided scale with the population.

The yellow line on the right represents revenue in millions of pounds, measured on the same scale as population. Just before this he writes that the red line on the left represents population in millions, and that the scale, one division per million, is numbered “at both ends.”

Population and revenue are measured on one and the same scale. The numbers appear at the left and right edges of the chart so that either can be read from both sides. It is not a dual axis, with the left for population and the right for revenue.

Turning the text’s explanation into a table of data and appearance gives the following. The example is France (before the Revolution), with figures from the table on p.33.

Data	Appearance	Example: France
Extent of the country (square miles)	Size of the circle; arranged from left in descending order of size	Circle
The extent figure	Number inside the circle	149,000
Population (millions)	Height of the red vertical line to the left of the circle	26
Revenue (millions of pounds)	Height of the yellow vertical line to the right of the circle, on the same scale as population	19
Weight of tax relative to population	Which way the dotted line joining the tops of the two vertical lines rises	Falls to the right
Population density (people per square mile)	Number directly above the circle	174
Maritime or land power	Color of the circle; green for maritime powers, pale red for land powers	—

The number above each circle is population density, not revenue. The text itself gives examples: “5 over Russia, and 14 over Sweden.” If you read France’s 174 as revenue, it is wildly at odds with the table’s £19 million.

In the French translation’s plate, the circles are colored blue and red. France, Spain, Britain and others are blue; Poland, Prussia and others are red. The maritime powers that the English text calls “green” can be read as having become blue in the French translation.

Only the circles for Russia and Turkey are divided inside. In Russia the inner circle is European territory and the outer ring is Asian territory; Turkey is split into sectors by continent. The text does not explain this division. Here the colors stand for continents rather than maritime or land power. It can be read as the colors carrying two different meanings within a single chart.

The circles of the Russian Empire and the Turkish Empire

Making Countries’ Shapes into the Same Circle

Why draw countries as circles? Playfair gives his reason on p.15.

for where forms are not similar, the eye cannot compare them easily nor accurately. From this circumstance it happens, that we have a more accurate idea of the sizes of the planets, which are spheres, than of the nations of Europe which we see on the maps

Where forms are not similar, the eye cannot compare them easily or accurately; so we have a more accurate idea of the sizes of the planets, which are spheres, than of the nations of Europe that we see on maps. In the preface, p.6, he also writes that when the eye sees similar forms it cannot help comparing their sizes.

On how the size of a circle is determined, the text does not say “proportional to area.” The preface’s phrase is “proportional circles.” I measured the plate of the French translation (the image above, 2000px wide): Russia’s circle has a diameter of about 410px and France’s about 68px. Taking France’s diameter as 1, area-proportional circles would give Russia about 5.6, and diameter-proportional circles about 31. The measured ratio is about 6, so the diameter is roughly proportional to the square root of the extent. Even allowing for measurement error, this is a value consistent with area proportionality.

The circles are arranged from left to right in order of size. On p.7 Playfair writes that on seeing this chart one will be struck by the size of Russia and Turkey. He continues that one will also be struck by the smallness of the nations that have moved the world these five hundred years.

He also decides what not to draw. On p.15 he writes that he combined extent, population, and revenue in one chart because they are the foundation of a power that can be accurately measured. He goes on to say that the form of government and the virtue of individuals also affect a nation’s strength, but cannot be shown in a chart.

The Direction of the Dotted Line Divides at “One Pound per Person”

The explanation of the dotted line spans p.13 to p.14.

The dotted lines drawn between the population and revenue, are merely intended to connect together the lines belonging to the same country. The ascent of those lines being from right to left, or from left to right, shews whether in proportion to its population the country is burdened with heavy taxes or otherwise.

The dotted lines are there to connect the two lines belonging to the same country. And which way a dotted line rises shows whether the country is burdened with heavy taxes in proportion to its population. Read only the first half, and the dotted line is merely a connector. It is the second sentence that gives the dotted line meaning.

Because population and revenue are on the same scale, a dotted line rises to the right when the revenue figure exceeds the population figure. That is, it is a country where tax per person exceeds one pound. Let us check against the tables in the text (£1 = 20 shillings).

Country	Population (millions)	Revenue (millions of pounds)	Tax per person in the table	Dotted line
Russia	25	7.3	6s 8d	Falls to the right
France	26	19	14s 8d	Falls to the right
Spain	11	14	£1 5s 5d	Rises to the right
Britain and Ireland	14	27	£1 18s 3d	Rises to the right

The figures are from the tables on pp.19, 33, 35 and 37. In the chart, the dotted line rises to the right for only four countries: Spain, Britain and Ireland, Portugal, and the United States. Those are also the only four countries whose tax per person in the tables exceeds one pound.

The section with Poland, France, Spain, and Britain and Ireland. France’s dotted line falls to the right, those of Spain and Britain rise to the right, and Britain’s rises most steeply

Detail cropped from Chart 1, original drawing by William Playfair, engraved by Adam (French translation, 1802). Source: David Rumsey Map Collection, David Rumsey Map Center, Stanford Libraries / Internet Archive (CC BY-NC-SA 3.0)

It matters that Playfair speaks only of the direction in which the line rises. On a common scale, direction works correctly as a binary split at “one pound per person.” The text’s explanation can be read as promising that much and no more.

The steepness of the slope is a separate matter. The two vertical lines stand touching the left and right edges of the circle, so the horizontal gap between them is the circle’s diameter. The slope of the dotted line is the difference between revenue and population divided by that diameter. Russia has a large difference, but its circle is large, so its dotted line is gentle. Britain has a difference of 13 and a small circle, so its dotted line rises steeply. The country’s extent gets mixed into the steepness of the slope.

How Third Parties Saw the Chart

The David Rumsey Map Collection’s catalog entry for the French translation quotes a comment by Akerman and Karrow. Their view is that the circles for countries’ extent, the sectors of Turkey, and the lines for population and taxes are “a creative combination of different visual forms.”

It was Pritchard (2021) who criticized the slope. He sums up the novelty as three things: the area-proportional circles, the division of Russia and Turkey, and the vertical lines with the sloping lines joining them. He then writes:

a direct comparison of these lines, one country to another, is not possible because each gradient is affected by the radius (and hence the area of the country)

The point is that because the slope depends on the circle’s radius, that is, on the country’s extent, it cannot be compared from one country to another. Jonathan Sachs makes the same point in a note, naming Funkhouser (1937) and others. Incidentally, Pritchard writes that population is read on the left scale and the budget on the right, which conflicts with the text’s “the same divided scale.”

Sachs’s main argument is in the history of ideas. He gives weight to the replacement of irregular national territories with “uniformly scaled circles.” If wealth is measured as revenue, a quantity, then wealth is not the same as the extent of territory. He reads this chart as a rebuttal to those who held that territory and population are wealth.

Juliana Adelman (2018) of The Irish Times focuses on intent. She writes that what Playfair most wanted to show was the advantage of the United Kingdom, which raised large revenue from a small territory and a small population. She then adds that overseas colonies are not counted.

Uses

There are four things to take away from Chart 1.

The first is to put things into the same shape before comparing sizes. On a map, countries’ extents are hard to compare. Playfair replaced them with identical circles and left the work of comparing to the eye.

The second is that putting two quantities on the same scale lets you read the relationship between them. By putting population and revenue, which have different units, on one numerical scale, he made a dividing line of “one pound per person” appear on the chart.

The third is that if you show a derived quantity by slope, you should keep the horizontal gap constant. In Chart 1 the gap is the circle’s diameter, itself data, and the country’s extent got mixed into the weight of tax. If you want to show tax per person, drawing that value directly is more reliable.

The fourth is to put the instructions for reading beside the chart. In Chart 1, the reading instructions exist only in the text. If you cut out the chart alone, you could easily misread the population density above a circle as revenue.

On p.16 Playfair writes that the greatest criterion is what remains in the mind of someone who looks at it for the first few minutes. By my count, Chart 1 tried to show six things in those few minutes: extent, population, revenue, population density, maritime or land power, and the weight of tax. The slope of the dotted line can be read as the awkward part that this cramming produced.

References

Primary sources

William Playfair, The Statistical Breviary; Shewing, on a Principle Entirely New, the Resources of Every State and Kingdom in Europe, London, 1801 (Bodleian Library copy, digitized by Google). Internet Archive / Full text / Google Books
William Playfair (trans. D.-F. Donnant), French translation (Paris, 1802), Chart 1, “Carte statistique représentant l’étendue, la population et les revenus des principales nations de l’Europe”. David Rumsey Map Collection. Copy on the Internet Archive

Secondary sources

Chris Pritchard, “Life of Pie: William Playfair and the Impact of the Visual,” Mathematical Association, Presidential Essay No.4, July 2021. PDF
Jonathan Sachs, “1786/1801: William Playfair, Statistical Graphics, and the Meaning of an Event,” BRANCH. Article
Juliana Adelman, “The man with a finger in many pie charts,” The Irish Times, 2018-03-05. Article
Catalog entry of the David Rumsey Map Collection for Chart 1 of the French translation (quoting Akerman & Karrow). Copy on the Internet Archive

The Same Square Miles, Drawn as the Area of a Circle and as the Angle of a Sector: Playfair in 1801 and 1805

Sun, 12 Oct 2025 00:00:00 +0900

William Playfair (1759–1823) drew the same quantity, a country’s extent in square miles, in two ways. In his 1801 chart, he drew it as the area of a separate circle for each country. In his 1805 chart, he drew it as the angle of a sector cut from a single circle. In his own words, the former is meant to show the sizes of countries to the eye, and the latter the proportions among the extents of the provinces.

The 1801 chart is Chart 1 of Playfair’s book The Statistical Breviary. The 1805 chart appears in Statistical Account of the United States of America, Playfair’s English translation of a book by D. F. Donnant. Following the two original books, both readable on the Internet Archive, this article looks at what each circle lets the eye compare and what it does not.

1801: Lining Up Countries by the Size of Circles

Photograph of the plate of Chart 1 in The Statistical Breviary

The Statistical Breviary appeared in London in 1801. Its title page says it is accompanied by stained copper-plate charts. Chart 1, in the text’s own description, shows the countries of Europe before the French Revolution. The image below is the same chart from the French translation published the following year.

Chart 1 in the French translation. The large concentric circle at the far left is Russia, and the circle divided into sectors to its right is Turkey. The circles for the countries get smaller toward the right

The explanation on p.13 of the text is brief. Each circle stands for one country, the countries are arranged in order of extent, and the number inside each circle is that country’s square miles. The preface calls this a way of representing the sizes of countries with “proportional circles.” The reason for using circles is given on p.15.

The present charts are in like manner intended to aid statistical studies, by shewing to the eye the sizes of different countries represented by similar forms, for where forms are not similar, the eye cannot compare them easily nor accurately.

Show the sizes of countries in similar forms: where forms are not similar, the eye cannot compare them easily or accurately. Countries on a map have irregular shapes, he continues, so we have a more accurate idea of the sizes of the planets, which are spheres.

In the text’s table, Russia is 4,650,000 square miles and Turkey is 790,000. Turkey is 0.17 times Russia. When I measured the diameters on the French translation’s plate, Russia came to about 411 pixels and Turkey about 171. The ratio of areas is 0.17, which nearly matches the ratio in the text. The ratio of diameters is 0.42. From this measurement, the circles can be read as drawn so that area, not diameter, is proportional to square miles.

How to read the vertical lines for population and revenue and the like is left to the article reading Chart 1 of 1801.

Cutting Inside the Circle: Russia’s Ring, Turkey’s Sectors, Germany’s Overlapping Circles

The circles of the Russian Empire and the Turkish Empire

In Chart 1, only Russia and Turkey have their circles divided inside. Russia’s circle is double: the inner circle is “European Dominions” and the outer ring is “Asiatic Dominions.” Turkey’s circle is divided by lines from the center into three parts: “European,” “African,” and “Asiatic.”

Turkey’s circle. Lines from the center divide it into three sectors: Europe, Africa, and Asia

Source: William Playfair (1801) / Wikimedia Commons (scan from the Cambridge University Press 2005 reprint). Public domain

The figures for this breakdown appear nowhere in the book. When I measured the French translation’s plate, Russia’s inner circle was about 24% of the area of the outer circle, and Turkey’s European sector was almost a quadrant. But with no numbers to compare against, there is no way to check whether the divisions match the actual extents.

In Chart 2 of the same book (Europe after the Peace of Lunéville), the ways of using circles multiply. According to the explanation around p.50, the whole of the German Empire, the Emperor’s (Austria’s) dominions, and Prussia’s dominions are drawn as three overlapping circles, A, B and C. Red is Austria’s share, yellow is Prussia’s share, and only the remaining green is the German Empire in the true sense. France’s circle is surrounded by a pale red circle that represents the combined extent of the countries under its control.

Part of Chart 2 in the French translation. France’s circle is surrounded by an outer circle, and three circles are drawn overlapping below Germany’s circle

Source: David Rumsey Map Collection, David Rumsey Map Center, Stanford Libraries / Internet Archive (part of Chart 2 of Donnant’s French translation Élémens de statistique, 1802, cropped). CC BY-NC-SA 3.0

In the 1801 book alone, there are at least three ways of dividing within a circle: concentric circles (Russia, France), sectors (Turkey), and overlapping circles (Germany). In Russia and Turkey, the circle’s area remains the extent of the whole country, and a further division is laid over its interior. Germany’s overlapping circles are a different matter. Chris Pritchard (2021) sees this as a diagram of relationships that pays no attention to size (“relationships with carefree attention to scale; they are simply representational”).

Playfair himself does not write why Russia gets a ring and Turkey gets sectors. Pritchard presents one possibility: a theory from an anonymous blog by a company called Geolytix. The circle was small, with no room to color Turkey’s three parts as a triple of concentric rings, so he may have divided it radially just this once. Turkey is the second-largest country in Chart 1, so the point of the story is not the smallness of the country but that there were three parts.

1805: Cutting Up a Single Circle

The plate of the 1805 circle of the United States

In 1802, Donnant published Élémens de statistique in Paris, a French translation of The Statistical Breviary to which he added an account of the United States. Playfair translated that part into English and published it in London in 1805 through Greenland and Norris. According to the translator’s preface, the translation is not literal, and he supplemented it with a “divided circle.” The title page says this circle was drawn “BY A NEW METHOD,” and beneath the figure is the following sentence.

This Newly invented Method is intended to shew the Proportions between the divisions in a Striking Manner

That is, a new method meant to show the ratios among divisions in a way that catches the eye.

The 1805 circle of the United States. The pink sectors in the upper half are the states, the green sectors at the bottom and right are Louisiana and the Western Territory, and the white sector at the left is East and West Florida

Source: William Playfair / Lehigh University Libraries, Special Collections. No Copyright – United States

Below it is “Total Extent 1,929,000 Square Miles or 1224 Millions of Acres.” The extent of the whole United States is given by this sentence, not by the size of the circle.

The extent of each state and region is expressed by the central angle of its sector. They run counterclockwise in descending order of size, starting from Virginia at upper right. According to the Lehigh University exhibit’s explanation, the colors distinguish three things: states, territories, and the East and West Florida held by Spain. Florida is an uncolored sector labeled “East & West Florida not yet united.” The newly acquired territory is drawn as Louisiana, marked “Newly Acquired (in 1804).” The figure on its sector is 725,000 square miles, about 38% of the total. Taken at face value, the angle would be about 135°.

On p.17 of the text, the purpose of the circle is stated as follows.

The divided circle which represents the surface of the country, has been constructed for the purpose of shewing, in an easy and accurate manner, the proportional extent of the different provinces…

It was made to show the proportions among the extents of the provinces simply and accurately. Right after this he goes on to say that the proportions of Virginia and Delaware are too out of balance. This is a comparison of part with part inside the circle. A little earlier on the same page is a sentence saying that even a third-rate province (a third rate province) is as large as France with its enlarged dominions. This comparison with Europe is made in the text, not in the circle.

Note that the data in this figure contains errors. According to David Bellhouse, New York and New Jersey are confused, and the area of each state was obtained by multiplying length by width. The details are covered in the article reading the 1805 United States pie chart.

What the Two Circles Let You Compare, and What They Do Not

The table below shows which appearance corresponds to the same square miles in the two charts.

Data	1801 Chart 1 (countries of Europe)	1805 chart (United States)
Extent of a country, state, or region	Area of a separate circle for each country	Central angle of a sector within a single circle
The extent figure	Written inside the circle	Written inside the sector
Extent of the whole	—	The sentence under the figure; the size of the circle does not represent it
Breakdown	Concentric circles for Russia, sectors for Turkey (no figures)	All sectors
Color	Green = maritime powers, pale red = land powers	Distinguishes states, territories, and East and West Florida

By my own sorting, what the eye can compare divides as follows.

	1801 circles of quantity	1805 divided circle
Number of circles / size of the circle	One per country / data	One / not data
Easy to compare	One country’s extent against another’s	Part with whole, part with part
Cannot be told from the figure alone	Each country’s share of the whole; the values of the Russia and Turkey breakdowns	The ratio of the United States’ extent to other countries'

The 1801 chart can be read as a chart for comparing one country with another. But how much of Europe as a whole Russia takes up cannot be known without adding up all the circles.

The 1805 chart can be read as the reverse. At a glance you can see that Louisiana’s sector is the largest division. But whether the United States is larger or smaller than Russia cannot be told. If drawn on the 1801 scale, the United States’ circle would come to about 0.41 times Russia’s in area and about 0.64 times in diameter. The comparison with France being written in the text can be read as due to the fact that this circle cannot show it.

The 1805 circle takes the size of the circle out of the data and gives meaning only to the divisions. Bellhouse sees Playfair in 1801 as not fully aware of what he had done. From that view, the “newly invented method” of 1805 can be read as pointing to making the angle of the sector the lead.

How Third Parties Saw the Two Circles

Pritchard calls the 1801 Turkey circle the first pie chart. The Wikipedia “Pie chart” entry takes the same position. Pritchard also asks why there were two ways of dividing. He treats the view that likens Germany’s overlapping circles to the later Venn diagram as a reasonable one.

David Bellhouse (2022) draws a different line.

In terms of an actual pie chart, Playfair was not fully aware of what he had done at that point. His fully-fledged pie chart, which appeared in 1805…

His position is that the 1801 chart was only a forerunner and that the real pie chart is the 1805 chart. He then points out the errors in the data seen above.

In the abstract of “No Humble Pie” (2005), Ian Spence writes that the origin of the pie is unclear and that the idea most likely came from the logic diagrams of Llull, Euler, and others.

On p.14, Playfair wrote that the eye compares sizes with almost unrivaled accuracy. Later perception research has produced different results. William S. Cleveland and Robert McGill (1984) proposed a hypothesis that ranks the accuracy of reading a quantity in this order: position on a common scale, length, angle, area, volume. Both the area of 1801 and the angle of 1805 fall on the side that is harder to read than position or length.

Drew Skau and Robert Kosara (2016) examined what the eye relies on in pie charts.

Data is encoded in pie and donut charts in three ways: arc length, center angle, and segment area… Both studies point to angle being the least important visual cue for both charts

Data is encoded in three ways, arc length, center angle, and segment area, and angle turned out to be the least effective cue. When a single circle is cut up, the three have the same ratio, so the proportions do not change whichever you read by.

However, what Playfair was comparing were the irregular shapes of countries on a map against uniform circles. The later studies compare circles with bars and the like. Because the questions differ, they can be read side by side rather than against each other.

Uses

The first is to decide, before drawing, what the reader is to compare with what. If you want items compared with one another, line up separate shapes as in 1801. If you want to show proportions within a whole, cut up a single shape as in 1805. Which one you choose swaps the comparisons that are visible with those that are not.

The second is that if you draw by area, make the area proportional to the data. If you make the diameter proportional, Turkey looks 0.42 times Russia rather than 0.17 times.

The third is that if you layer two roles onto one circle, you should show the interior values too. The Russia and Turkey breakdowns have no figures, so the reader cannot check whether the divisions are right.

The fourth is to be aware that in a divided circle, the size of the circle represents nothing. Add the value of the whole in a sentence, and prepare another means for comparison with outside countries. And even if you draw the angles accurately, errors in the original data are not fixed.

References

Primary sources

William Playfair, The Statistical Breviary; Shewing, on a Principle Entirely New, the Resources of Every State and Kingdom in Europe, London: J. Wallis et al., 1801. (Preface, pp.13–16, the chapters on Russia and Turkey, explanation of Chart 2 around p.50) Internet Archive / OCR text
D. F. Donnant, Statistical Account of the United States of America, translated by William Playfair, London: Greenland and Norris, 1805. (Title page, translator’s preface pp.v–vi, p.17) Internet Archive / OCR text
Lehigh University Libraries, Special Collections, “Statistical Representation of the United States of America” (images of the 1805 chart and title page) Exhibit item / Exhibit “At a Glance” commentary
Plates of Donnant’s translation Élémens de statistique (Paris, 1802), David Rumsey Map Collection (mirrored on the Internet Archive): Chart 1 / Chart 2
Wikimedia Commons, “File:Playfair-piechart.jpg” (Turkey’s circle of 1801) Commons

Secondary sources

Chris Pritchard, “Life of Pie: William Playfair and the Impact of the Visual,” Mathematical Association Presidential Essay No. 4, 2021. PDF
David Bellhouse, “William Playfair’s Statistical Graphs,” CSHPM Notes / Notes de la SMC, 54(1), 2022. PDF
Ian Spence, “No Humble Pie: The Origins and Usage of a Statistical Chart,” Journal of Educational and Behavioral Statistics, 30(4), 2005, pp. 353–368. (Abstract) IDEAS/RePEc / doi:10.3102/10769986030004353
Drew Skau and Robert Kosara, “Arcs, Angles, or Areas: Individual Data Encodings in Pie and Donut Charts,” Computer Graphics Forum, 35(3), 2016, pp. 121–130. eagereyes
William S. Cleveland and Robert McGill, “Graphical Perception: Theory, Experimentation, and Application to the Development of Graphical Methods,” Journal of the American Statistical Association, 79(387), 1984, pp. 531–554. (The ranking is taken from lecture materials) Cornell INFO 3312 lecture slides
Ian Spence and Stephan Lewandowsky, “Displaying Proportions and Percentages,” Applied Cognitive Psychology, 5(1), 1991, pp. 61–77. (Abstract) Introduction by the ISU Graphics Group
Wikipedia, “Pie chart” (History section) Wikipedia / “William Playfair” Wikipedia