Tuesday, July 28, 2009
Just say no to perspective pie charts!
Several papers in a row tripped over one of my pet peeves. Why does anyone who cares about their data use a perspective pie chart?
Pie charts in general get little respect in the visualization community (Tufte hates them), but while I don't love them I don't hate them either. The scheme is intuitive and widely understood -- the area (and pie angle) of each slice is proportional to its share of the total. This is one class of objection: area is a harder visual concept to compare than lengths. The other is that these waste dimensions and pixels -- why use two dimensions and lots of pixels to display information which can be shown in one dimension with many fewer pixels. While pixels are not gold, why not maximize their value?
But there's no excuse for perspective pie charts. These abominations are made easy by programs such as Excel, whose defaults tend to range between poor graphical taste and appalling graphical design. The perspective view completely ruins the correlation between shape and value, killing the one virtue of a pie chart.
So, the next time you review a paper with one of these horrors, object! Journal editors of the world: ban them from your pages. Data deserves respect!