Quant plot hotfixes

[gidden]

These are two small hotfixes I needed in the past months but never was able to push. I will keep this PR in draft form to remember to update it if I find they are still necessary on the latest main.

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Name of contributors Added to AUTHORS.rst
• Description in RELEASE_NOTES.md Added

Adding to RELEASE_NOTES.md (remove section after adding to RELEASE_NOTES.md)

Please add a single line in the release notes similar to the following:

- (#XX)[http://link-to-pr.com] Added feature which does something

Description of PR

Please describe the changes introduced by this PR.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (19d08a5) 94.5% compared to head (10ed090) 94.8%.
Report is 29 commits behind head on main.

Files	Patch %	Lines
pyam/compute.py	50.0%	1 Missing ⚠️
pyam/plotting.py	0.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #809     +/-   ##
=======================================
+ Coverage   94.5%   94.8%   +0.3%     
=======================================
  Files         59      62      +3     
  Lines       6016    6043     +27     
=======================================
+ Hits        5686    5730     +44     
+ Misses       330     313     -17     

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