Spie chart -- a comparison of two pie charts

A pie chart depicts a partition. A Spie chart combines two pie charts
to compare partitions. One pie chart is drawn as-is, and serves as the
basis for comparison. The other is superimposed on the first, using
the same angles for the slices, but different radii, so as to achieve
the desired areas.

The example shows road casualties data from Israel. The base pie chart
shows the general population parititoned into gender and age
groups. The superimposed chart shows the same partitioning for the
population of road casualties. Obviously the main age group hit is
20--24, and males are much more often involved in accidents that females.

Reference:
D. G. Feitelson, "Comparing Partitions with Spie Charts".
Technical Report 2003-87, School of Computer Science and Engineering,
The Hebrew University of Jerusalem, Dec 2003.
URL http://www.cs.huji.ac.il/~feit/papers/Spie03TR.pdf