Make code work with updated dependencies

[MarieRoald]

Make the code, docstrings and tests work for updated dependencies.

The main change is due to an update in Matplotlib that affected how we got style information for the threshold values in the outlier plot.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.99%. Comparing base (f1ecd6f) to head (737c4c5).

Files with missing lines	Patch %	Lines
tlviz/visualisation.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #25      +/-   ##
==========================================
- Coverage   95.04%   94.99%   -0.05%     
==========================================
  Files          13       13              
  Lines        1472     1478       +6     
==========================================
+ Hits         1399     1404       +5     
- Misses         73       74       +1     

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