UsableMarkets

Lately I’ve had pie charts on the brain. For some reason they get abused in the most unfortunate ways. An excellent description of how a pie chart should function appears in Information Graphics, A Comprehensive Illustrative Reference, by Robert L. Harris.

“The major purpose [of a pie chart] is to show the relative sizes of components to one another and to the whole.”

A nice diagram helps explain.

Too often, though, computer programs offer us nothing at all what the proper pie chart should be like.

Power Point often gives us something bulky, like this.

It just hurts even to look at it.

Charting software, which is usually used to generate charts for websites and dashboards often produce pie charts like this, as seen on socialPicks.

The result is meaningless gobbledygook.

But pie charts actually have a rich history. They are one of the four major chart types (along with bar charts, line charts and scatter plots), and were invented by William Playfair, the father of statistical graphing, sometime around the 1790′s.

An example of one of William Playfair’s pie chart diagrams.

Here he uses several in a sequence to compare populations of various cities. (This chart is from Edward Tufte’s great book, The Visual Display of Quantitative Information.)

But there is a severe problem with pie charts. To quote Howard Wainer, in his book Graphic Discovery, “the … pie chart comes to mind as an obvious example of flash over experience.”

Tufte is more blunt. From The Visual Display of Quantitative Information he says: “A table is nearly always better than a dumb pie chart; the only worse design than a pie chart is several of them, for then the viewer is asked to compare quantities located in spatial disarray both within and between pies.”

A glance at the Playfair example above should be enough to see Tufte’s point. A table would have been a far better choice by Playfair. But hey, he was inventing.

And I happen to agree with Tufte, not just because he is right, but because most people just don’t know how to design a pie chart in a sensible way. If you ever think you might want to use a pie chart, just don’t.

But to every rule there is of course an exception.

Thanks to Seen and Not Seen (and some guy named Chris D.) for this one.

As always, thanks for listening.
~alex

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