I came across this pie chart from a presentation at an industry meeting some weeks ago:
This example breaks a number of the unspoken conventions on making pie charts and so it is harder to read than usual.
Notice that the biggest slice starts around 8 o'clock, and the slices are ordered alphabetically by the label, rather than numerically by size of the slice.
The following is the same chart ordered in a more conventional way. The largest slice is placed along the top vertical, and the other slices are arranged in a clock-wise manner from larger to smaller.
This version is easier to read because the reader does not need to think about the order of the slices. The expectation of decreasing size is met.
The above pie chart, though, reveals breaking of another convention. The colors on this chart signify nothing! The general rule is color differences should encode data differences. Here, the colors should go from deepest to lightest. (One can even argue that different tinges is redundant.)
You see how this version is even better. In the previous version, the colors are distracting. You're wondering what they mean, and then you realize they signify nothing.
As designers of graphics, we follow a bunch of conventions silently. When a design deviates from it, it's harder to understand.