Pie charts may look pretty, but it’s difficult to spot differences between the sizes of the pie slices compared to bar charts.
A colleague pointed me to http://www.businessinsider.com/pie-charts-are-the-worst-2013-6 for an explanation.
Donut charts are an improvement over pie, but still not as good as bar charts.
http://hypsypops.com/pie-chart-donut-chart/