Fix layers 3 & above overlapping the axis lines

[has2k1]

fixes #798

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.14%. Comparing base (8a4881a) to head (324636c).

Files	Patch %	Lines
plotnine/themes/themeable.py	75.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #802      +/-   ##
==========================================
- Coverage   87.15%   87.14%   -0.02%     
==========================================
  Files         179      179              
  Lines       11628    11634       +6     
  Branches     2119     2121       +2     
==========================================
+ Hits        10134    10138       +4     
  Misses        991      991              
- Partials      503      505       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.