The purpose of a pie chart is to tell a story about the parts-to-whole aspect of your data. In other words, it describes how big one part is compared to other parts. This all sounds like a visualization with a good purpose, so why shouldn’t you use it?
Pie charts are made up of differently angled sections. These angles are what we are visually trying to interpret when we examine a pie chart. Below is a chart illustrating 5 categories that all look very similar in a pie chart and can be very difficult to distinguish from each other. When you look at the same values in a column chart, you can see how they are very different from each other.
A pie chart should only be used as a last resort and only in circumstances where a pie chart is easy to understand. In our example above, we are not able to distinguish between the values the pie pieces are supposed to represent. When we get the values into a different format, for example a column chart, the differences of the values become more apparent.