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it's worse than a pie chart because visually it's much harder to compare sizes of horizontal and vertical rectanglesResearch I've seen suggests the exact opposite -- that people are great at estimating area of rectangles, and terrible at estimating area of circular or triangular shapes. They know which circle is bigger, but don't have any idea by how much bigger.
That said, pie chart slices work for simple data not because of area, but because all you have to compare is the simple length of the circumference segments. That's a single dimension, and easy to compare.
See "Pizzas: or Square? Psychophysical Biases in Area Comparisons", http://groups.haas.berkeley.edu/marketing/PAPERS/PRIYA/p5.pd... for how people lean on a single dimension for size or area comparisons.
Unfortunately, as this cell phone OS chart is trying to support comparison of multiple sets and subsets, circumference segments alone are inadequate to convey the relative sizes of the sets.
The more complex the information, the more the usefulness of the nested rectangles versus pie chart slices becomes clear. Imagine for example a visual representation of drive space usage by directory and subdirectory.
Here is a drive space chart using nested rectangles:
http://grandperspectiv.sourceforge.net/ScreenShots/1_0-Folde...
For comparison, here is an attempt do do the same using pie charts:
http://www.daisydiskapp.com/img/d4.png
In fact, if you use both apps, you'll see DaisyDisk is not able to give you the "grand perspective" in a single view, it requires clicking to zoom in on a slice it then expands to a full pie to drill down.
