William Playfair

William Playfair is among the most important and influential people in the history of data visualization. He is widely credited with inventing several significant types of data visualizations around the turn of the nineteenth century, including the bar, circle, pie, line, and area charts. He distilled his motivation for creating data visuliastions instead of using tables:

As the eye is the best judge of proportion, being able to estimate it with more quickness and accuracy than any other of our organs, it follows, that wherever relative quantities are in question, a gradual increase or decrease of any revenue, receipt or expenditure of money, or other value, is to be stated, this mode of representing it is particularly applicable; it gives a simple, accurate, and permanent idea, by giving form and shape to a number of separate ideas, which are otherwise abstract and unconnected. In a numerical table there are as many distinct ideas given, and to be remembered, as there are sums, the order and progression, therefore, of those sums are also to be recollected by another effort of memory, while this mode unites proportion, progression, and quantity all under one single impression of vision, and consequently one act of memory.

William Playfair, Lineal Arithmetic Applied to Shew the Progress of the Commerce and Revenue of England during the Present Century (London:1789)

Playfair’s insight about the ability of graphical representations to convey ‘proportion, progression, and quantity’ resonates through the centuries. Indeed, this idea is central to understanding how data visualizations work and it anticipates a modern understanding of cognitive science, visual perception, and psychology.

William Playfair invented several now-ubiquitous data visualizations. This example combines two of them: the line chart and the area chart. This chart shows England’s imports and exports over the course of the eighteenth century. The long, steep decline in England’s exports and imports is immediately visible across the course of the 1770s: a period of economic, political, and military turmoil that included a minor disagreement with the colonies in America.

This is among Playfair’s most intriguing creations, and it contains several innovations in data visualization. The circles represent the principal nations of Europe, scaled in proportion to their geographic area (the first circle chart). The lines rising tangentially from each circle represent population on the left and revenue on the right. The lines share an ordinal scale, but the left scale is measured in millions of people and the right scale in millions of pounds (a dubious dual-axes mechanism). The first pie chart, Venn diagram, and nearly-a-doughnut chart are all here. Playfair, perhaps unaware of high-school geometry, argued that the slopes of the lines could indicate whether a nation was under- or over-taxed.