The Pie of Pie Chart is a built-in chart type in Excel. Pie charts are meant to express a "part to whole" relationship, where all pieces together represent 100%.
Pie charts work best to display data with a small number of categories (2-5). The Pie of Pie Chart provides a way to add additional categories to a pie chart without generating a pie chart too complex to read. When configuring a Pie of Pie chart, Excel provides a setting that moves the smallest n slices of the pie to another smaller pie, where n can be adjusted to suit the data.
Pie of pie charts should be avoided when there are many categories, or when categories do not total 100%. The human eye has trouble comparing the relative size of slices in a pie chart, and this problem is compounded with the pie of pie variety.
A simple way to handle more categories in a pie chart
Can be read "at a glance" with limited categories
Excel can calculate and display percentages automatically as data labels
Difficult to compare relative size of slices
Become cluttered and dense as categories are added
Pie charts are one of the simplest chart types in Excel, good for showing "part-to-whole" relationships with data in a small number of categories. Pie charts get a lot of criticism in the professional data visualization...