Robert Kosara returns with a great writeup on the pro’s and con’s of Pie charts, including some great science in human perception on how we analyze and understand them, as well as a great short-list of reasons why to use and to avoid them.
There are two features that let us read the values on a pie chart: the angle a slice covers (compared to the full circle), and the area of a slice (compared to the entire disk). Research suggests that we look at the angle in the center, essentially reducing the chart to just the crossing lines there. We are not very good at measuring angles, but we recognize 90º and 180º angles with very high precision. Slices that cover half or a quarter of the circle will therefore stand out. Others can be compared with some success, but reading actual numbers from a pie chart is next to impossible.
