Add coordinate system for stacked plots

[bvreede]

Closes #48

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 64.34%. Comparing base (0f6b43e) to head (7922194).

Files	Patch %	Lines
R/coord-stack.R	0.00%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
- Coverage   65.78%   64.34%   -1.45%     
==========================================
  Files          16       17       +1     
  Lines         491      502      +11     
==========================================
  Hits          323      323              
- Misses        168      179      +11     

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