On Linux, the smem tool can give the proportional amount of RAM that a process is using.
On embedded Linux, procrank can do the same thing.
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.