In what seems to be a post-vacation syndrome, I am in the mood for pie charts. I see them everywhere, even in car logos.

Actually, I am more in the mood to defy current “crowd wisdom” about pie chats.

Search the web for “pie chart” and you’ll get more than one million results, and a depressing picture of human knowledge. Browse the first 100 and what do you get? Some educational(?) sites poor kids), tutorials (Excel, php, java, Illustrator), humor (here, here), bad (here, here, here, here or here) or just plain stupid examples. You’ll also find them in in court or fighting government (who could ever imagine that?). I’ll leave for another post what the Wikipedia and the pie chart thread in Tufte’s Ask E.T. say about pie charts (Stephen Few’s Save the Pies for Dessert is not listed within the first 100 results).

An old litany

Some of these sites discuss the use of pie graphs, but they usually recite the same old litany: our perception is bad at judging angles, you should use no more than five or six categories, don’t use them to compare series, Cleveland’s findings, etc. (there also is at least one unfair comparison between pie and bar graphs and one very aggressive rant against them).

If there is something that I would like to have written about pie graphs it is this Expert notes at ManyEyes:

Pie charts have a mixed reputation. They are popular in business and the media but many information designers have criticized the technique. Some claim that the pie slice shape communicates numbers less exactly than other possibilities such as line length. But this remains unclear in the context of proportions: for example, we have seen no studies that looked at the task of judging whether an item is more or less than 50%. It’s also unclear whether exact communication of numeric values is the only evaluation criterion; at least one study indicates that use of a pie chart for analyzing a problem as opposed to a bar chart changes the way people think about the problem.

This is clearly more constructive than saying that “they are as professional as a pair of assless chaps” (less funny though).

Not all charts are born equal

Current wisdom presumes that bar graphs and pie graphs are equivalent. For that reason, bar graphs should be used, always. After all, they are more efficient, right? But if they are not equivalent, as the above quote suggests? Take a time series, for example. If you want to see trends, you’ll choose a line graph; if you want to compare data points you’ll use a column graph. They are very similar, but by choosing one or the other, the designer is making a choice of how he/she’ll  look at the problem. Bar graphs and pie graphs are very different, so shouldn’t we think twice before selecting a bar graph because of its presumed superior efficiency?

This disdain for pie charts has its roots in Cleveland’s work and in Tufte’s and Few’s writings. Their positivist view towards information visualization may be as relevant as the classic economic theory and its presumption that consumer always take the rational decision, but are we not all predictably irrational? I agree with Robert at EagerEyes when he says:

There is no doubt that we need to be careful about the choice of visual representation, and that we need to encourage the use of good charts and criticize the bad ones. But that doesn’t mean we can get lazy and squeeze everything into a few standard charts types we’ve been using for decades. That is especially true if we want people to actually care about what we’re trying to show – and not bore them to tears.

We should probably try to be more rational and circumspect in a decision-making environment and do not use the media as our role model, otherwise business visualization may become useless. However, ruling pie charts out is not the wisest decision.

Simple rules are made for beginners. Let’s break some. How about this one:  “you should use no more than five or six categories in a pie chart”. Are you sure?

(Before that, we must re-read what Cleveland said and what others said about Cleveland. That’s the next post.)