Pie & Donut Plots

[IPWright83]

There should be support for:

• PiePlot
• DonutPlot
• StackedDonutPlot

PiePlot could simply wrap the StackedDonutPlot and provide an innerRadius of 0 and an outer radius based on (width - margin.left - margin.right) / 2.

Need to consider labels however. Using the Legend is simple.

• Should we support call outs labels?
• Should there be an automatic "other" grouping for small values?