You’re getting a bit detailed for a comment thread; each answer will only lead to new questions. Derek is right: this is an area in which a lot of painstakingly detailed research has been done. Read Cleveland or Robbins or Tufte (who is my hero, in no small part because his prose is so beautiful). If you present numeric data visually you will communicate more clearly and more elegantly by taking their recommendations to heart.

This isn’t stuff that was arbitrarily made up one afternoon as a hoax to fool the rest of you. The professionals really do know what they’re talking about.

The ideal case for a pie chart is n=3, when each pair of segments is continuous, so you can easily assess combined shares and so on. In this case only, the pie might be better than the data.

In this case, n=3 (viz., %insured publicly, %insured privately, %uninsured) for 50 states. Even here, I don’t think pie charts would work very well.

That’s true in general of graphic representations of data, insofar as they’re less precise than the data itself.

Take party vote shares as an example.

Why not population statistics by year? They’re both roughly equally poor choices of data to represent using a pie chart.

Look, pie charts are reasonably good at doing two things simultaneously: giving a rough idea of the relative sizes of a collection of numbers, and giving a rough idea of the size of each number compared with the total. Sometimes that’s useful, and lots of times it isn’t.

For example, if the collection of numbers is really small—as in the electoral result example—then a graphic representation isn’t of much use in the first place. And if the numbers are too close together, then a representation that emphasizes their slight differences—such as a bar graph—works much better.

But if you want to represent, say, a budget—in which both the rough relative sizes of pairs of individual components, and the rough relative size of each component compared to the whole, are of interest, then a pie chart seems to me like a fine choice. And—wouldn’t you know it—that’s the kind of thing that pie charts have traditionally been used for.

Now, Kieran, you suggested that even in these cases, you’d rather use something else instead. I’m curious—what would you use? I can imagine, for instance, a rectangle subdivided into segments, as individual bars often are in a bar graph. But I don’t expect it would be any easier to visually compare nearly-equal subsegments of this rectangle than it would nearly-equal pieces of a pie chart. Am I wrong?

Not a pie chart, showing pattern of coalition deaths in Iraq by month.

With n>=4, the numbers can be a bit tricky, but the pie chart is even worse. For example, suppose the parties are ordered A,B,C,D and you want to judge whether A+C is a majority. With the numbers you have to do some arithmetic, but with only the chart it’s just about impossible, assuming the combined share is close to 50 per cent.

The ideal case for a pie chart is n=3, when each pair of segments is continuous, so you can easily assess combined shares and so on. In this case only, the pie might be better than the data.

Adobe Illustrator has built-in support for pie chart creation, so I’d say it’s actually more time-intensive to build a chart like the ones displayed here.

Labelling is likely easier too. And I suppose they’re easier to knock together than pie charts when you’re working to a deadline in Paintshop Pro, which is probably why they’re turning up.

I’m not sure Kieran’s points about arbitraryness is all that strong. The layout of the subcomponents a pie-chart is arbitrary. The layout of cartograms is arbitrary: there’s isn’t a unique approximation to geography.

You have a lot more options with these things, but that doesn’t mean your choice is going to be arbitrary. The data aren’t unidimensional. Some categories are more similar than others, and you can represent that by their locations in 2D space. Basically making them cartograms in category space rather than geographical space. With pie charts and stacked bars you can only do this in 1D space.

Fidelity, btw, has a mosaic type presentation of market activity that is handy. Rectangles and their size represent companies and market share while number of shares traded is presented in color saturation. Deceiving, maybe, in that it draws attention to actively traded shares, but effective as a graphical presentation, especially with hyperlinks.