A pie chart is useful when you want to show relative proportions or contributions to a whole. A pie chart uses only one data series. Pie charts are most effective with a small number of data points. Generally, a pie chart should use no more than five or six data points (or slices). A pie chart with too many data points can be very difficult to interpret.
The values used in a pie chart must all be positive numbers. If you create a pie chart that uses one or more negative values, the negative values will be converted to positive values—which is probably not what you intended!
Overview of pie chart subtypes:
The basic pie chart everyone knows and loves, which shows the breakup of a single series of data.
The name sounds like a Vaudeville gag, but the exploded pie chart simply separates each piece of a pie with a small amount of white space. Usually, Excel charting mavens prefer to explode just a single slice of a pie for emphasis. This technique uses the ordinary pie subtype, as explained in the next chapter.
Pie of Pie
With this subtype, you can break out one slice of a pie into its own, smaller pie (which is itself broken down into slices). This chart is great for emphasizing specific data; it's demonstrated in the next chapter.
Bar of Pie
The bar of pie subtype is almost the same as the pie of pie subtype. The only difference is that the breakup of the combined slice is shown in a separate stacked bar, instead of a separate pie.
Pie in 3-D and Exploded Pie in 3-D
This option is the pie and exploded pie types in three dimensions, tilted slightly away from the viewer for a more dramatic appearance. The differences are purely cosmetic.