Spot diagram: Add ray energy or wavelength as plot colour

added enhancement label

changed milestone to %v0.4

added low priority label

created branch 123-spot-diagram-add-ray-energy-or-wavelength-as-plot-colour to address this issue

assigned to @y.zobus

mentioned in merge request !128 (merged)

mentioned in commit 5ae2dafb

closed with merge request !128 (merged)

mentioned in issue #206 (closed)