For vtkPlotBar, SetInputArray can be used to identify input
series' beyond index 1.  When this is done, each additional
series will be plotted on top of the previous series as a
stacked bar plot.

To handle this change, vtkPlot now handles multiple labels that
can be associated with each of the plots.  PaintLegend and
GetNearestPoint have been adjusted to deal with this properly.