Fixing plot indexing

[germa89]

While working in #2471 I found out that test_plot_incomplete_nodal_selection was not passing, not sure what did I change in that PR (I check I promise).

Hence I'm bringing the fix I did for that in a separate PR to highlight its importance.

I have also extensively documented the approach so the future developers (me) know what I did.

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[codecov]

Codecov Report

Merging #2476 (8423731) into main (d89968a) will increase coverage by 0.04%.
Report is 3 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2476      +/-   ##
==========================================
+ Coverage   83.76%   83.81%   +0.04%     
==========================================
  Files          44       44              
  Lines        8660     8660              
==========================================
+ Hits         7254     7258       +4     
+ Misses       1406     1402       -4     

[germa89]

Needed, so ... LGTM.

[pyansys-ci-bot]

✅ Approving this PR because germa89 said so in here 😬