Fixes bug 13796 where bottom-right plots pan badly

An additional special case is needed in the vtkChartXY panning code for
when a plot is in the bottom-right corner. This would work as expected
assuming anything was in the top left or right corner and so could be a
little trickier to replicate.

Change-Id: Ia960cd2734d13055cdaddd4646f00756170ee7fe
Merge topic 'chartxy-plot-corner-bug' into master

1ff34b63 Fixes bug 13796 where bottom-right plots pan badly