BUG #14599: Handle double values properly.

Plots used to rendering transfer functions failed to work correctly when
the values/ranges they were presenting were in the double realm. This
was because these classes didn't handle vtkPlot::ShiftScale at all. This
fixes that issue.

Change-Id: I08c487c6e735b5a7055daff7e537e97036449e96
Merge topic '14599_fix_double_range_issues' into master

9085e579 BUG #14599: Handle double values properly.