2D Histogram - Option to invert color scale or use standard Qgis gradient widget

[jo-chemla]

Describe the solution you'd like
It would be great to add an option to 2D Histogram chart made to Invert Color Scale, or to let the user chose histograms using the standard Qgis colorramp/gradient widget.

Describe alternatives you've considered
Some color scale for the 2D Histogram are inverted by default (like the grayscale with black indicating highest bin count), but some others are great (eg BlueToRedSoft). It would be good to have the option to edit the histograms color scales the same way we can do it within qgis, with the default gradient editor widget - plus add the ability to invert the selected color scale.