Colorbar under histogram

[pllim]

Description

This pull request is to add a colorbar to build on #2498. Alas, it is not as easy as when using matplotlib. Problems:

• Visually, added the colorbar object in figure.axes shows it in pop-up but I cannot figure out how to make it appear in Plot Options without popping things out. Not sure which figure axis number to adjust here.
• Visually, not sure how to make the ColorAxis span entire plot width. bqplot examples do not dive so deeply and not obvious from API doc.
• Would be nice if I can just use plugin.viewer.selected_obj._composite_image.scale_image as the ColorScale for ColorAxis but it does not seem to faithfully represent what glue is rendering in the image viewer.
• In ColorScale (that is actually from yet another package called bqscales), scheme follows Colorbrewer names, not glue names. So, "gray" in glue does nothing. I have to use "Greys" as understood by Colorbrewer. Where is the mapping between glue and Colorbrewer for this?
• In ColorScale, scale_type only allows "linear" (according to bqscales), so the colorbar refuses to update when non-linear function is given. Forcefully setting plugin.stretch_histogram.figure.axes[2].scale.scale_type = "sqrt" would throw TraitError: The 'scale_type' trait of a ColorScale instance expected any of ['linear'], not the str 'sqrt'.

As it currently stands, I am actually not sure if we can use this without upstream fix(es).

Refs:

• https://bqplot.readthedocs.io/en/stable/_generate/bqplot.axes.ColorAxis.html
• https://bqplot.readthedocs.io/en/stable/_generate/bqplot.scales.ColorScale.html
• https://github.com/bqplot/bqscales/blob/master/bqscales/scales.py

Somewhat related PR(s):

• EXP: Add colorbar to image maybe glue-viz/glue-jupyter#403

Change log entry

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  should add a no-changelog-entry-needed label.

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[maartenbreddels]

I'm wondering if it makes sense to use the ColorScale of bqplot, and instead create a small bqplot figure and draw the image ourselves. I think glue does most of the value->rgb mapping itself, and I think it's going to be difficult to get feature parity between the ColorScale and Glue.

[pllim]

Turns out I am in the wrong rabbit hole. SMEs said we should not use ColorScale. Kyle suggested alternative using matplotlib + histogram colors=. I want to keep what I have here for future reference, so I am going to close this and open a new PR.