Wednesday, July 20, 2011

death to pie charts

I hate pie charts. 

I mean, really hate them.

Those who have heard me speak on data visualization will have learned that the only thing I hate more than a pie chart is a 3D, exploding pie chart - they are the absolute worst - but the plain vanilla pie charts are pretty bad, too. Here's a recent one from TechCrunch, which is intended to show how much they cover start-ups versus big companies (full article):


I'll start with the lesser evil of the above visual: meaningless color. The pie above is what happens if you put the data in Excel and say "chart data". I've said this before and I'll say it again: graphing your data with a tool like Excel should be the first step in your design process, not your last! In TechCrunch's pie, the color itself doesn't represent anything, it's simply used as a categorical differentiator. One unintended side effect is the optical illusion you get with a darker colored slice appearing larger than a same-size slice of a ligher color.

My strong opinion is that color should always be an explicit choice and should be used strategically to draw the audience's eye. This preattentive power is being wasted here. If you must use a pie chart, at least make the slices the same color and highlight only the one or two you want to draw attention to. Or if you don't want to highlight a particular slice, but rather are intending the visual to aid in information discovery, you may consider something like the following:

Hopefully you can see that this still isn't a very good visualization. The labels are messy. Only a few things are immediately apparent: General Consumer Web is the biggest piece, there are a lot of small slices.

My main beef with pie charts like the one above (and in general) is this: our eyes aren't good at attributing quantitative value to two dimensional spaces. In English: pie charts are really hard for people to read! When segments are close in size, it'd difficult (if not impossible) to tell which is bigger. When they aren't close in size, the best you can do is determine that one is bigger than the other, but you can't judge by how much. To get over this, you can add data labels, as they've done in the TechCrunch version. But I'd still argue the visual isn't worth the space it takes up.

What should you do instead? My typical advice would be to replace a pie chart with a horizontal bar chart, organized from greatest to least or vice versa (unless there is some intrinsic value in the categories, in which case that should be followed). With bar charts, our eyes compare the end points. Because they are aligned at a common baseline, it’s very easy to assess relative size. This makes it easy to see not only which segment is the largest (for example), but also how incrementally larger it is than the other segments. Here's what this looks like with the TechCrunch data:


One might argue that you lose something in the transition from pie to bar. The unique thing you get with a pie chart that is absent in a bar chart is the concept of there being a whole, and thus, parts of a whole. But if the visual is difficult to read, is it worth it? Ultimately, it's up to the designer of the visual. My advice is as follows:
  1. Don't use pie charts.
  2. If you find yourself unable to follow #1, keep in mind the challenges with pie charts: if relative sizes are important, you'll need to include data labels. Also be aware of impact of color on 2D space (darker looks larger); don't let your tool decide your color scheme. 
Personally, I will continue to avoid pie charts.

3 comments:

  1. The only good pie chart is a chart of pies. I like Blueberry.

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  2. I tried combining the two together and blogged on it, but at the end of the day, the standard horizontal bar always works the best.

    My post:
    http://2toria.com/2011/07/21/messing-with-charts-etc/

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  3. Your points are right on, and I agree in 90+% of cases.

    There are a few circumstances where I think a pie chart communicates quite effectively. Most often when I'm trying to emphasize that a particular subset - say, a customer segment - is either a very large or very small component of the whole. In those cases, I think the visual impact of the pie is stronger than using bars.

    That said - you're right that pie abuse runs rampant!

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