Pie chart

A pie chart turns relative frequencies into areas: the whole circle is 100 percent of the observational units, and the slice areas add to $1$. For example, if 40 of 200 students choose band, that category takes $40 / 200 = 0.20$ of the circle, a slice of $0.20 \times 360 = 72$ degrees. Pie charts only make sense when the categories are mutually exclusive and cover every individual exactly once. Comparing slices by eye is harder than comparing bar heights, so a bar graph is usually the clearer display.