Fix categorical plotting

[melonora]

This PR fixes #228.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.01887% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 77.71%. Comparing base (602c27a) to head (f12947c).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #229      +/-   ##
==========================================
+ Coverage   75.30%   77.71%   +2.40%     
==========================================
  Files          11       11              
  Lines        1543     1660     +117     
==========================================
+ Hits         1162     1290     +128     
+ Misses        381      370      -11     

Files	Coverage Δ
src/spatialdata_plot/pl/render.py	94.26% <90.00%> (+4.74%)	⬆️
src/spatialdata_plot/pl/utils.py	66.55% <78.78%> (+3.84%)	⬆️