Thursday, November 3, 2011

Friends Don't Let Friends Use Pie Charts

It's time for a little chat. I'm not going to name any names here, but some of you have been using these to represent your data:

Worse than that, a few of you have been using these:

Look, I know how it is. You start off with an x- and y-axis, and a few data points. Later on, line graphs just aren't good enough anymore. You move into bar charts and start colour-coding.  Then Excel comes out with an updated version with even more features and you're in a multimodal haze with all the things you can do. Before you know it, you're hooked.

And you know you've hit rock bottom when a pie chart is your "go to" graph. I know you're better than that. Sure, there are times when a 2D version is appropriate, but it's time to face some cold hard facts. It's time for an intervention.

Pie charts work best when comparing the proportions of two categories. But the problem with pie slices is that it is difficult for users to compare the area of the slices. If you just need to communicate something very basic ("A is bigger than B"), it's probably okay. However, if you need users to understand details (or change over time), then a bar chart is a better graphic. People are much more accurate at reading length, rather than wedges. Labels are simpler to apply. (Although this is kind of an ugly example of pie vs. bar, it does make a good point about why bar graphs are better.) Take a look at this redesign of a pie chart to bar graph on Junk Charts for a good illustration of some of the issues.

As for 3D pie charts? Um, hey, who has three variables that they're trying to plot on a pie graph? (If you do have three, there are other---better---ways to show your data.) Not you. Annie Pettit's Prezi pretty well sums things up on this point. (Hat Tip to Jon Peltier for this.)

So, are we cool? I'm really glad we had this little talk.

1 comment:

  1. Agree with you re 3D pie charts - as a mathematician. But if I was DELIBERATELY trying to skew opinion in favour of one value (heaven forbid that I would make data lie), I'd use the 3D chart every time. Spin doctors LOVE 3d pie charts. I mean, you don't want people to really try to ANALYSE what you're telling them, do you?