Render stacked plots as separate colors in legend

Modified a test to verify functionality, and fixed the PaintLegend
method to use the appropriate color for the separate series in the
stacked plot.

Change-Id: I2d38aaa93b33e329736266bcbb90b44b273f49e7
Merge topic 'charts-stacked-plot-legend' into master

ffd2444c Render stacked plots as separate colors in legend