Derived class method signature differs from superclass signature.

GCC compilers warn (-Woverloaded-virtual)
virtual vtkStdString vtkPlot::GetTooltipLabel(const vtkVector2d&, vtkIdType, vtkIdType)' was hidden by
virtual vtkStdString vtkPlotHistogram2D::GetTooltipLabel(const vtkVector2f&, vtkIdType, vtkIdType

vtkPlot contains the correct signature for the method. Replaced
vtkVector2f with vtkVector2d.

Change-Id: Id10b0630660a3babb9662add2e98defa18494ad3