ChartPie's tooltip is formatted in
vtkChartPie::LocatePointInPlot. This method relies on
PlotPie:::GetNearestPoint, but the plotpos vector was not being
computed by GetNearestPoint. Consequently, the tooltip always reported
a 0 value for the dependent variable.

In this patch, PlotPie:::GetNearestPoint computes the dependent
variable and return it in plotpos[1]. In addition, plotpos returns the
independent vaiable in plotpos[0]. ChartPie now uses plotPos.GetY() to
obtain the dependent variable. This seems consistent with other
charts.

To reproduce the problem (before the patch) and verify (after the
patch) run:

vtkChartsCoreCxxTests "TestPieChart" -I
and move the mouse of each pie chart segment.

Change-Id: I5c3d56a79c3ceca6616bae2b0a7a85f9c15f2293