Add plot column pair utility method

[katxiao]

Resolves #223

[codecov-commenter]

Codecov Report

Base: 69.56% // Head: 70.18% // Increases project coverage by +0.61% 🎉

Coverage data is based on head (405340b) compared to base (4a79665).
Patch coverage: 96.66% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #224      +/-   ##
==========================================
+ Coverage   69.56%   70.18%   +0.61%     
==========================================
  Files          70       70              
  Lines        2425     2482      +57     
==========================================
+ Hits         1687     1742      +55     
- Misses        738      740       +2     

Impacted Files	Coverage Δ
sdmetrics/reports/utils.py	97.98% <96.61%> (-0.93%)	⬇️
sdmetrics/reports/__init__.py	100.00% <100.00%> (ø)

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[fealho]

Looks pretty good! Left a couple of minor comments, good to go otherwise ⚡