Information visualization specialists like Stephen Few and Howard Wainer often call out the 3-D Pie Chart as a graphical pariah. The curved areas of pie charts are already difficult to compare already, and they become worse when a 3-D perspective is added. Though authors like to rail on it, I suspected the form didn’t really occur in serious or even semi-serious data presentations. However, below is such a graph I found in the wild recently.
It’s from an North Carolina School of Science and Mathematics summary of alumni giving, and the graph breaks it down by class year. There seems to be one only piece of information to be gleaned from the graph: some class gave a lot more than any others. You can’t tell which one, because the classes of 1984 and 1993 are assigned the same color wedge. The graph may fall into the so-bad-it’s-good category because it tells you that there is something interesting in the data, forcing you to read the provided table of numbers to figure out just what it is. Unfortunately, the report has no explanation for the 1993 spike, but I’m guessing it was a single .com jackpot winner.
Here’s a quick scatterplot of the same data with 1993 removed (so it wouldn’t throw off the scale) and a Loess smoother added. It’s not surprising to see that older alumni give more money. The 1985 and 1987 mini-spikes are partially explained by considering class sizes. As I recall, the class size went from about 150 students to about 250 with the class of 1985, and the school’s two grades alternated between big and small sizes for a few years until they either evened out or the school grew again and expanded the even year class sizes to catch up.