Chartjunk
As with bar charts, pie charts are often graphical representations of a small number of values. For example, a pie chart of the gender of students in a class is only based on a single value, the proportion of males. As a result, there is a temptation to 'enhance' pie charts as 3-dimensional objects — chartjunk.
Resist the temptation — it does not make the data any easier to understand and may indeed be misleading since 3-dimensional pie charts can over-emphasise the categories closest to the viewer.
Kestrel deaths
The 3-dimensional pie chart below shows causes of death of kestrels in Britain between 1963 and 1997.
The viewpoint tends to make the closest categories appear too large. In particular, Disease incorrectly appears to be as common a cause of death as Unknown. (There were 77 deaths caused by Disease and 114 of Unknown cause.)
Small is beautiful
In general, it is better to draw a standard pie chart smaller rather than embellishing it with chartjunk.
Marital status on first birth
As part of a study in the USA of trends in the marital status of women when they had their first child, data from the 1995 Current Population Survey were examined. The 'exploded' pie chart below describes those who had their first child when aged 15 to 29.
The simpler small pie chart below shows the data more clearly.