Fix is_optimal sometimes being used for y_metric_name

[mpolson64]

Summary:
When generating the plot the y_metric name is inferred based on the other columns expected to be. When is_optimal was added this needed to be updated as well.

Now, instead of inferring we're just going to supply the y metric name directly -- this should be less error prone.

Differential Revision: D64123183

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.72%. Comparing base (367dd2c) to head (2546c29).
Report is 197 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2849      +/-   ##
==========================================
- Coverage   95.72%   95.72%   -0.01%     
==========================================
  Files         500      500              
  Lines       49127    49126       -1     
==========================================
- Hits        47028    47027       -1     
  Misses       2099     2099              

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