Circle graphs are sometimes called pie charts or pie graphs. They are used to represent data that is part of a whole, for example percentages and fractions. These percentages or fractions are represented at a certain point in time, and are not changing.
To view the different parts of a circle graph, open the following link:
http://nces.ed.gov/nceskids/help/user_guide/graph/pie.asp
Notice that circle graphs do not have a x-axis or a y-axis, so no labels are needed.