A whole-to-part plot shows how the parts make up the whole.
A pie chart is a common representation which shows a circle divided into sectors for each class, with the angle of each sector proportional to the frequency.
A stacked bar plot shows rectangular bars for each class with the size of each bar proportional to the frequency.
The major problem with the pie chart is that it is difficult to judge the angles and areas of the sectors. It is useful if you want to compare a single class relative to the whole. For comparing classes to each other, we recommend the frequency plot or stacked bar plot for greater clarity and ease of interpretation.