fix: Form data through loop over channels for multichannel coupled histo notebook

[kratsg]

Pull Request Description

Resolves #1313.

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• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

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* Use pdf.config.channels API instead of the channel names from the spec.
* Use pyhf.contrib.viz.brazil API for visualization.
* Deprecate use of pylab in notebook.

Co-authored-by: Matthew Feickert <matthew.feickert@cern.ch>

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Codecov Report

Base: 98.26% // Head: 98.26% // No change to project coverage 👍

Coverage data is based on head (cdcdf70) compared to base (cbd68c8).
Patch has no changes to coverable lines.

Additional details and impacted files

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##           master    #1974   +/-   ##
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  Coverage   98.26%   98.26%           
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  Files          68       68           
  Lines        4440     4440           
  Branches      728      728           
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  Hits         4363     4363           
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	27.22% <ø> (ø)
doctest	60.96% <ø> (ø)
unittests-3.10	96.19% <ø> (ø)
unittests-3.7	96.17% <ø> (ø)
unittests-3.8	96.21% <ø> (ø)
unittests-3.9	96.23% <ø> (ø)

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[alexander-held]

Looks good to me, I am fairly sure this fixed channel order is how this was always meant to be.