Changed color mapping method to support TF's vector mode for vtkChartHistogram2D
Changed color mapping method, from MapScalarsThroughTable2 to MapVectorsThroughTable, to support color transfer function vector mode.
If you have a vtkImageData having a scalar array that have 2 components - let's say a real and an imaginary part - and that you use a color transfer function with a "magnitude" vector mode, with this fix, the image data will be plot correctly in the vtkChartHistogram2D.