Pie Charts – A Devil’s Advocate Point of View

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.)

9 thoughts on “Pie Charts – A Devil’s Advocate Point of View

  1. I fall fairly heavily in the no pie chart camp.

    One instance where they are ok in my opinion would be the pie charts on the left of the website fivethirtyeight.com

    They still have the typical downsides of needing the numbers next to the pie chart if you want to get a fairly accurate estimate of the exact amount in each category and I think the information is better displayed if you used a bar chart instead. But the point of these charts is more of a “who is ahead?” and “is it close?” than wanting to know if the exact number is 270 or 272 etc.

    From a “it is prettier” perspective or “it fits in the corner of my annual corporate report nicely”, or “I just want variety in my charts” then one can argue for pie charts at times, but for best display/utility of the information I have yet to see a pie chart out perform other options.

    So what I would like to see (from anyone) is a case where a pie chart can be argued to display the information better than another choice.

  2. I don’t quite see where the devil comes in, you have only shown that pie charts are in wide use and that some people like them and some don’t. But I’m looking forward to more posts in this series, this should be interesting.

  3. Use as many categories as you want, plus a 3-D vantage:
    Some stats stuff (picture with pie chart + extra per-time stats).

    (I’m just spitting back one of the more creative examples from your web search of “pie chart”.)

  4. @Jon: Yes, I saw that nice pie too. Let’s see if there are better examples of pie charts with more than five slices.
    @Robert: What I want to say is that there is a place for pie charts as an information visualization tool (the devil’s advocate comes in because that’s not the official (Tufte’s) dogma).
    @Scott: I’ll do my best to find a case…

  5. Using the same type of chart to represent a certain relationship would be boring. However Pie charts have some limitations and some strong voices against their use. What other options could be used then? – a dot plot. other options?

  6. The problem with a stacked chart is that it is as difficult to compare stacked items to each other (because they lack a shared baseline) as it is to compare segments of a pie.

  7. I think that chart formats are not interchangeable, so if you want to see a part-of-the-whole you must use a circular chart or a stacked bar. I’ll discuss this in my next post and I’ll try to show how we can make a better use of pie charts. Stay tuned!

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