Pie charts show proportions within a whole. The slices are subcategories of a single category. Slices add up to 100% or 1.
• Avoid using pie charts if all the slices are similar in size.
• Limit pie charts to eight slices or less (Wong, 2010).
• Label directly on the pie slices, rather than using a legend.
• Keep pie slices the same color. Use the whitespace between slices to differentiate the slices.
Sosulski, Kristen. Data Visualization Made Simple (p. 50). Taylor and Francis. Kindle Edition.