Ensure stacked plots show cumulative legend

[scottbell]

Closes #7403

Describe your changes:

Listen to series models in the legend, and hide options that don't make sense if the cumulative legend isn't being used.

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• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Is this change backwards compatible? For example, developers won't need to change how they are calling the API or how they've extended core plugins such as Tables or Plots.

Author Checklist

• Changes address original issue?
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Reviewer Checklist

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[scottbell]

Note, composables would be really good here, but I'm a bit reluctant to refactor heavily in a bug fix.
Also note this currently contains the changes from the other legend PR I've got.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b33a017) 55.37% compared to head (1e83ed3) 48.93%.
Report is 2 commits behind head on 7403-plot-legends-legends-not-expanding-when-requested.

Additional details and impacted files

@@                                    Coverage Diff                                     @@
##           7403-plot-legends-legends-not-expanding-when-requested    #7481      +/-   ##
==========================================================================================
- Coverage                                                   55.37%   48.93%   -6.44%     
==========================================================================================
  Files                                                         671      428     -243     
  Lines                                                       26935    13329   -13606     
  Branches                                                     2614        0    -2614     
==========================================================================================
- Hits                                                        14914     6522    -8392     
+ Misses                                                      11305     6807    -4498     
+ Partials                                                      716        0     -716     

Flag	Coverage Δ		*Carryforward flag
e2e-full	48.93% <ø> (ø)		Carriedforward from fbaaad5
e2e-stable	?
unit	?

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see 501 files with indirect coverage changes

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[ozyx]

Changes look and work great, just a small suggestion