test: update causality plot tests, minor bug fix

[skim0119]

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[codecov-commenter]

Codecov Report

Merging #81 (491bfd2) into main (4320dcf) will increase coverage by 3.58%.
The diff coverage is 69.23%.

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
+ Coverage   61.65%   65.24%   +3.58%     
==========================================
  Files          35       35              
  Lines        1098     1099       +1     
  Branches      123      124       +1     
==========================================
+ Hits          677      717      +40     
+ Misses        406      366      -40     
- Partials       15       16       +1     

Flag	Coverage Δ
unittests	65.24% <69.23%> (+3.58%)	⬆️

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Impacted Files	Coverage Δ
miv/visualization/__init__.py	100.00% <ø> (ø)
miv/statistics/spiketrain_statistics.py	87.75% <20.00%> (-7.81%)	⬇️
miv/statistics/pairwise_causality.py	100.00% <100.00%> (+64.28%)	⬆️
miv/visualization/causality.py	100.00% <100.00%> (+70.83%)	⬆️

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