Tuesday, September 16, 2008

Pie charts do not speak clearly; they just mumble

Gene Zelazny’s textbook Say It with Charts notes that: “In general, pie charts are the least practical of the five chart forms. They also are the most misused and, worse, the most abused.“

In an eloquent 14-page newsletter article titled Save the Pies for Dessert Stephen Few also argues that pie charts are not a very useful tool for presenting information. Quantitative comparison of percentages on a pie chart requires comparing angles that are spatially in disarray. That is more difficult for people than looking at a simple horizontal bar chart and comparing distances.

Pie charts only work for large differences, which geeks call semi-quantitative comparisons. If you want a quantitative comparison with a pie chart, then you have to mark the percentages on it, so you might as well use a table (or a bar chart). A whole row of pie charts is even worse than one pie chart.

Adding a third dimension and shading just makes a pie chart prettier but harder to interpret. Making it transparent (like you can in MS Excel) reduces it to “dancing bearware” – a gee-whiz effect that add nothing but hoopla.

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