process/core/io/plot missing docstrings and raised exceptions

[clmould]

Description

fix the improve-quality-job missing docstrings and missing raised exceptions in the process/core/io/plot files

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 49.30%. Comparing base (6dbb302) to head (947b603).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4498   +/-   ##
=======================================
  Coverage   49.30%   49.30%           
=======================================
  Files         151      151           
  Lines       29611    29611           
=======================================
  Hits        14599    14599           
  Misses      15012    15012           

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[timothy-nunn]

Just one very picky comment