Concerning my post on the Economist's use of donut charts, Phillip on Blogcritics raised two issues worth further study.
First, are pie and donut charts "natural" for representing percentages? In my opinion, if only one set of percentages is involved, then either a table of numbers or a "decile chart" works much better.
This "decile chart" addresses the visual estimation issue Phillip brought up. Because each dot / human figure represents 10%, even if the percentages are not annotated, the reader can gauge them visually. Not so for pie slices: no one will be able to tell a 15% slice from a 10% slice.
Besides, the point of the original graphic was to compare percentages. The message is that the white population is expected to decline while the Hispanic, Black and Asian populations would increase. When two pies (or donuts) are used, the reader is tasked with differentiating a 67% slice from a 58% slice situated an inch apart. That, I submit, is a tall order. By contrast, the growth rate is explicitly coded into the gradient of the lines in my junkart: the steeper the line, the higher the growth (or decay).
Second, what if growth rates are chaotic and lines criss-cross each other? This presents no problem at all:
The first line chart shows two segments increasing at the same rate and one segment declining fast. The second chart shows two segments dropping at different rates and one segment skyrocketing. Because the growth rate is explicitly plotted, the reader has no problem picking it up.
The astute reader will note this chart looks like the marvellous Bumps chart.
Phillip also noticed another feature of the Economist chart that escaped me: the two donuts were sized proportional to the total populations in 2004 and 2030 respectively. Ouch! Now, the area of a slice depends on both angle and radius, making it nigh impossible to compare them.
Thanks Phillip for pushing my thoughts on this.