Add last_plot() function to retrieve the most recently rendered plot

[has2k1]

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[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.82%. Comparing base (68b000a) to head (1b73fab).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
plotnine/themes/theme.py	16.66%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1039   +/-   ##
=======================================
  Coverage   86.82%   86.82%           
=======================================
  Files         202      203    +1     
  Lines       13783    13802   +19     
  Branches     1701     1702    +1     
=======================================
+ Hits        11967    11984   +17     
- Misses       1270     1273    +3     
+ Partials      546      545    -1     

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