Improve plot in GUI: Add r^2 and fit line toggle

[PinkShnack]

Issue from #48

Todo:

• update changelog
• add chi^2 annotation
• make the fit line toggable with the default visible

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.88%. Comparing base (505a7bc) to head (d6304fa).

Files with missing lines	Patch %	Lines
pyjibe/fd/mpl_indent.py	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
+ Coverage   67.79%   67.88%   +0.09%     
==========================================
  Files          36       36              
  Lines        2816     2824       +8     
==========================================
+ Hits         1909     1917       +8     
  Misses        907      907              

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[PinkShnack]

The toggle is here under "Plot", with the chi^2 in the plot.

[PinkShnack]

Hey @paulmueller looks like the tests do pass, but the win py311 had a strange error. if you could rerun it that'd be great.