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Came across an interesting post by Andy Kriebel on pie charts. He essentially deconstructs the decision-making process around how to choose the right format for showing change in a few variables over time. Sound simple? Yes, then no. What starts out as a simple exercise…

Andy Kriebel's pie chart exercise
pie chart showing three data points for one year; same format for another year... see how the data changes

…turns into a practical, instructive seven-step journey through different formats. As designers, we’ve all been through this process of trial-and-error, pros and cons. But it’s very helpful to see the breakdown of what you gain or lose by each format.

If a pie chart makes you squint, look away.

It’s pretty clear (at least to me) that the consensus is this: pie charts are good for one thing, and one thing only: showing the relationship of the parts to the whole. They are not good for showing a gazillion slices of data–they are good for showing two. Or three. They are not good if they make you squint. And they are useless for an apples-to-apples approach.

And donut charts? Forget about it. The white circle in the middle makes it impossible to see where the angles meet, undermining a user’s attempt to size up slices and make comparisons. EagerEyes cites research showing the readers focus on the center, where the angles are formed and the lines meet.

As EagerEyes points out, human beings are terrible at gauging anything other than 90 and 180 degree angles–we can discern halves and quarters but beyond that, not so much. That’s where bar charts and other formats come in. Stephen Few tells us to save pies for dessert.

My takeaway is: be very clear about what you are trying to show, and only use pie charts when:

  • it is important that you show the relationship of one thing to another that a pie chart intuitively portrays–how the parts relate to the whole (e.g., when a piece shrinks the other pieces grow)
  • AND (not if) you have a few points to share, not itty bitty slices that require squinting
  • you remember that pie charts do not do a good job of visually comparing and contrasting specific values to one another over time (an apples-to-apples approach

(Pssst… I’m still going to make donut charts now and then. They’re pretty.)