Add IndexedStackColorMapper for ImageStack glyph

[ianthomas23]

This is an early-stage WIP to add an IndexedStackColorMapper which is an alternative use of ImageStack glyph that color maps and displays a single 2D array of the 3D ImageStack data.

The data displayed is controlled by an index property, which will cause data[:, :, index] to be displayed. The actual colormapping is controlled by a contained single_color_mapper so that it can be linear, logarithmic or eq_hist. There is a boolean property map_all (maybe should be scan_all but I don't like either much) that controls whether the colormap limits/properties are calculated from the all of the 3D data or just the 2D data displayed.

Example script:

from bokeh.models import IndexedStackColorMapper, LinearColorMapper, Slider, Toggle
from bokeh.plotting import column, figure, row, show
import numpy as np

ny, nx, nstack = 5, 5, 4
rng = np.random.default_rng(22529)
data = rng.random((ny, ny, nstack))
for k in range(nstack):
    data[:, :, k] += k*0.2

p = figure(width=500, height=400,
           title=f"ImageStack of shape {data.shape} with IndexedStackColorMapper")

# Do not want palette on both LinearColorMapper and IndexedStackColorMapper as only one used ???
single = LinearColorMapper(palette='Spectral10')
color_mapper = IndexedStackColorMapper(single_color_mapper=single, palette='Spectral10')
im = p.image_stack(image=[data], x=0, y=0, dw=1, dh=1, color_mapper=color_mapper)

cbar = im.construct_color_bar()
p.add_layout(cbar, "right")

slider = Slider(start=0, end=nstack-1, value=0, step=1, title="Index")
slider.js_link("value", color_mapper, "index")

toggle = Toggle(label="map_all")
toggle.js_link("active", color_mapper, "map_all")

show(column(p, row(slider, toggle)))

and video of this:
https://github.com/bokeh/bokeh/assets/580326/ccfcc2cf-7c58-425e-9023-2019bf4bec9c

One problem with this approach is that both the LinearColorMapper and IndexedStackColorMapper must have a palette specified (None is not allowed for ColorMappers) but only one is used.

Also, I have had to reimplement part of ContinuousColorMapper._v_compute within IndexedStackColorMapper when it scans all of the data but only displays one 2D slice of it. Probably a better approach would be a refactor of the various _v_compute functions to accept the scanned data rather than scan it themselves.

[codecov]

Codecov Report

Merging #13316 (1465975) into branch-3.3 (4544dbd) will increase coverage by 0.00%.
Report is 2 commits behind head on branch-3.3.
The diff coverage is 100.00%.

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##           branch-3.3   #13316   +/-   ##
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  Files             315      315           
  Lines           20150    20156    +6     
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+ Hits            18629    18635    +6     
  Misses           1521     1521           

[bryevdv]

@ianthomas23 just an FYI in case you were thinking of 3.5 for this work, it looks like that release will be late May/early June (I think this could be nice to show off at Scipy particularly, but understand if there is not time permitting)

[ianthomas23]

@bryevdv I don't think it is likely that I'll ever finish this, so if someone else wants to take it over they are welcome to do so.

[bryevdv]

I'll keep the branch but close the PR, and open an issue for the future