Thursday, March 17, 2016

3-D Pie Charts are the Spawn of Satan

I have been reading and greatly enjoying Cole Nussbaumer Knaflic’s excellent recent book Storytelling with Data: a data visualization guide for business professionals. Her second chapter is about Choosing an Effective Visual. Under the heading  of To Be Avoided she has a subheading that pie charts are evil. She shows why on page 62 with an example similar to the following one I did in Excel. Based on a quick glance at the 3D pie chart, we’d like to answer these and similar questions:

Which supplier has the largest market share?

What percent is it?

Which supplier is second?

It looks like Supplier B (red) has the largest share, between a quarter and a third of the market.

 But, that’s an illusion resulting from a low-angle perspective. When we label the sectors with their percentages, it turns out that Supplier D (purple) really has the largest share with 35% rather than 31%. (Cole’s example had the two largest sectors next to each other, which made it simpler to compare angles. Mine doesn’t).

A plain, two-dimensional pie chart is slightly better. But, we still wind up mentally constructing what is shown more clearly in the following table:

Either a horizontal bar chart or a vertical bar (column) chart will do a better job of letting us visualize what is going on. That is because we can compare the ranked bar lengths against a common origin.
In this blog I typically use horizontal bar charts for presenting results from fear surveys, since they provide more room for captioning each bar with a long survey question.   

In a post on her Storytelling with Data blog titled The great pie debate she showed a slide for a talk on DEATH TO PIE CHARTS! I wholeheartedly agree.

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