Add color mapping to the bar chart

[mweiden]

Finishing up #1035

Changes made on top of @Donaldpherman's PR:

• Only color histograms that are selected for colorby
• Reuse binning functions from util/dataframe/histogram.js; this fixes an issue with d3 generating near-zero width bins
• Fix/simplify histogram color mapping functions
• Do not attempt to plot bins if the calculated binWidth is zero; this can happen if all dataframe column values are the same
• Refactor the function that draws the histogram a bit

Illustration of fixes to color mapping are below. Each shows the legend scaled against the colored histogram bins.

Before (source @liaprins-czi):

After (source this PR):

[codecov]

Codecov Report

Merging #1218 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1218   +/-   ##
=======================================
  Coverage   59.39%   59.39%           
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  Hits         2787     2787           
  Misses       1813     1813           
  Partials       92       92

Flag	Coverage Δ
#backend	45.59% <ø> (ø)	⬆️
#frontend	74.92% <ø> (ø)	⬆️
#javascript	74.92% <ø> (ø)	⬆️
#python	45.59% <ø> (ø)	⬆️
#smokeTest	?
#smokeTestAnnotations	100% <ø> (ø)	⬆️
#unitTest	59.39% <ø> (ø)	⬆️

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[liaprins-czi]

Looks great to me; thanks Matt!

[codecov-io]

Codecov Report

Merging #1218 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1218   +/-   ##
=======================================
  Coverage   59.40%   59.40%           
=======================================
  Files          64       64           
  Lines        4707     4707           
  Branches      374      374           
=======================================
  Hits         2796     2796           
  Misses       1819     1819           
  Partials       92       92           

Flag	Coverage Δ
#backend	45.52% <ø> (ø)
#frontend	75.00% <ø> (ø)
#javascript	75.00% <ø> (ø)
#python	45.52% <ø> (ø)
#smokeTest	?
#smokeTestAnnotations	100.00% <ø> (ø)
#unitTest	59.40% <ø> (ø)

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[bkmartinjr]

Brief testing shows that this PR has introduced a bug with brush display in histograms.

Test case:

1. load a dataset (I used TM_facs, and will refer to it throughout - although the bug should manifest in any dataset)
2. brush select a continuous selection (I did n_counts):
   

Note that the main graph shows the subset of selected cells.

3. Click color-by (droplet) button on same histogram. Note that the brush selection is gone, but that the cells are (correctly) still selected in the main graph.
   

The cells should stay selected (color-by changes do not modify the selection). What is very likely happening is that the changes to componentDidUpdate() fail to correctly recognize when the brush needs to be re-rendered.

I have noted the line where this bug is located.