[RTM] refactor: Plot

[Ipuch]

The refactor.

---

This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 81.61435% with 41 lines in your changes missing coverage. Please review.

Project coverage is 77.90%. Comparing base (f3b330a) to head (7d709cf).
Report is 12 commits behind head on master.

Files with missing lines	Patch %	Lines
bioptim/gui/plot.py	81.53%	41 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #932      +/-   ##
==========================================
+ Coverage   77.79%   77.90%   +0.10%     
==========================================
  Files         157      157              
  Lines       18071    18133      +62     
==========================================
+ Hits        14059    14126      +67     
+ Misses       4012     4007       -5     

Flag	Coverage Δ
unittests	77.90% <81.61%> (+0.10%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[pariterre]

Reviewed 3 of 3 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @Ipuch)

[pariterre]

@EveCharbie
Is it okay?