Fix for categorical plot with a single point

[hanjinliu]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 78.94737% with 8 lines in your changes missing coverage. Please review.

Project coverage is 83.91%. Comparing base (cde1d7d) to head (596ab68).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
whitecanvas/layers/tabular/_box_like.py	55.55%	2 Missing and 2 partials ⚠️
whitecanvas/layers/group/band_collection.py	72.72%	2 Missing and 1 partial ⚠️
whitecanvas/backend/matplotlib/canvas.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #68      +/-   ##
==========================================
- Coverage   83.93%   83.91%   -0.02%     
==========================================
  Files         198      198              
  Lines       22453    22476      +23     
  Branches     3712     3717       +5     
==========================================
+ Hits        18846    18861      +15     
- Misses       2636     2641       +5     
- Partials      971      974       +3     

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