When you want to illustrate shares or percentages of the total for 4-9 main categories. A pie graph with more categories if OK, if each is very small, but otherwise you have visual clutter, and the pie graph does not readily impart information.
A pie graph is used when representing portions of a whole.
if your trying to show percentage and not change in percentage u use a pie graph
For any data that can be best expressed as percentages of a whole, a pie chart or pie graph is appropriate.
pie graph
A pie graph is useful for showing percentages. It is great for showing percentages in a survey.
When dealing with percentages %
Line graph
A pie graph is more effective that other types of graphs for showing shares of a total.
A pie graph would be used for percents (anything that adds up to 100%)
In statistics when comparing data
On a pie graph
A pie graph does not have an axis.