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.

Predators and free-range poultry

The diagram below shows deaths of free-range chickens by predators during a study in France. The predators were classified into Carnivores (mainly foxes and dogs), Birds (mainly buzzards, goshawks and crows) and Unknown. The 'exploded' pie chart below describes the data.

The simpler small pie chart below shows the data more clearly.