Exploring Data Visually - Data Points: Visualization That Means Something - page 489

FIGURE 4-6 Bar graphs for survey results

PARTS OF A WHOLE

When you put categories together, the sum of the parts can equal a whole. Count
everyone in all the states and you have a national aggregate; combine sports
divisions and you have a league. Seeing categories as a single unit can be ben-
eficial if you want to see distributions or the spread across a single population.

Note: You might not be able to get the exact
value from a pie chart, but you can still make
comparisons when there aren't a lot of categories.

This is when the pie chart comes into the picture. A full
circle represents 100 percent of a whole, and each wedge
is a portion of that 100 percent. The sum of all the wedges
equals 100 percent. Angle is the visual cue.

Discussions on whether the pie chart is useful end up running in circles, so
you decide if you want to use them. If you do use pie charts, they tend to
clutter quickly when you have a lot of categories, simply because there is only
so much space in a circle, and small values end up as slivers.