Daily Archives: 2013-07-22

Every Now and Again, a Pie can be Good

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It is hard to find anyone in visualization today with much time for pie charts. In fact it seems de rigueur to disdain them. And yet we see an awful lot of them. Now, I’m not going to claim that they are a good, general purpose chart, but I do always like to think of times when a chart will actually work well.

When Pie Charts Work At All

One well-known requirement for a pie to have a chance of working is that the data represent a fraction of a whole. That’s the big selling point of pie charts — each data row should represent a fraction of the overall data. So pies work best for percentages and fractions, and second-best for counts, populations, weights — things for which there is a natural feeling that summing them all up and saying “that represents 100%” is good.

On the side of evil is when the numbers must not be summed — if the data represent means (for different sized groups) or degrees Fahrenheit, then a pie representation is flat-out wrong. It’s not a bad rule to say:

Only Use a Pie if it makes sense to think of the data values as summing to 100%

The second rule I’d suggest is based on the inability for people accurately to judge angles. Pies do not work well for that, so if you need accurately to judge numbers, do not use a pie. Pies work well for “A is about twice as big as B” or “ C is definitely smaller in the second pie”. They are not good for “C is very slightly lower than D” or “B is just under 33%”. Stating it positively:

Use a Pie if the goal is to make broad comparisons, not detailed ones.

Finally, I’d offer a third suggestion, rather than a rule. It’s based on the observation that a bar chart (a natural competitor to a pie chart) is very often improved by ordering — high to low values, for example. Pies can often look radically different when categories are re-ordered, and although it is sometimes suggested that you do this ordering for pies, I think that a pie for categories that can be re-ordered would almost certainly look better in another form. Instead I would suggest the following:

Use a Pie when the categories have a natural order

When Pie Charts Work Well

Stephen Few (Save the pies for Dessert: http://www.perceptualedge.com/articles/08-21-07.pdf) quotes a study showing that when pies have been shown to be actively superior to bar charts — it is when it makes sense to want to compare sums of categories (e.g. the sum of the first two against the sum of the second two); the reason being that in a pie, you can compare angles for multiple segments easily, whereas in a bar chart that is not easy. 

Survey Data: Bar Chart and Pie Chart

Survey Data: Bar Chart and Pie Chart

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