Monday, November 10, 2014

Pie Rules

"There is no data that can be displayed in a pie chart, that cannot be displayed BETTER in some other type of chart," is a quote Wikipedia attributes to the late, great statistician John Tukey, (I've found no original source for the quote). It gets worse when Excel and/or graphic designers start adding chart junk of 3-D projections in hope of adding more visual interest.
Any reasonable comparisons in the above chart are impossible.

If you insist on using pie charts, Benjamin Starr at Visual News offers some history and sensible guidelines for using pie charts.

First, as the lead-in graphic above illustrates, display no more than five categories in a pie chart. Many small areas are difficult to compare.
Second, since wedges of a pie are difficult to compare side-by-side, Starr says don't use multiple pie charts for comparison. Use stacked bar charts instead, as shown above.
Third, make sure that the percentages add up to 100% and that all slices are drawn proportionately to the percentages they purport to represent.
And finally, order the slices from largest to smallest, starting at 12 o'clock and continuing either clockwise or counter-clockwise to aid in comprehension.

Of course, we've posted some pie charts here and had some fun with them.