Visual Improvements for Separation Plot

[arshm06]

Description

Visually Improved the Separation Plot. (Issue number: #2114)

Pushed the expected events marker down. Moved the legend out of the plot. Remove the axes edgeline.

Checklist

• Follows official PR format
• Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• New features are properly documented (with an example if appropriate)?
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

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📚 Documentation preview 📚: https://arviz--2272.org.readthedocs.build/en/2272/

[codecov]

Codecov Report

Merging #2272 (6461dd1) into main (24c260a) will not change coverage.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2272   +/-   ##
=======================================
  Coverage   87.90%   87.90%           
=======================================
  Files         122      122           
  Lines       12459    12459           
=======================================
  Hits        10952    10952           
  Misses       1507     1507           

Files Changed	Coverage Δ
arviz/plots/backends/matplotlib/lmplot.py	98.75% <100.00%> (ø)

[OriolAbril]

Thanks again for the PR. There might have been an issue with the commits and/or branches. The PR looks identical to #2271, modifying plot_lm instead of plot_separation

[OriolAbril]

Closing for now, will reopen if updated with different changes