Fix inferred y orientation with computed x variable

[mwaskom]

I'm not totally satisfied with this, but mutating the layer dict
(the other option) doesn't seem much cleaner.

The result is proper behavior here:

so.Plot(tips, y="total_bill").add(so.Bar(), so.Hist())

[codecov]

Codecov Report

Merging #2818 (7dedde5) into master (651d06e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2818   +/-   ##
=======================================
  Coverage   97.96%   97.96%           
=======================================
  Files          67       67           
  Lines       21789    21800   +11     
=======================================
+ Hits        21345    21356   +11     
  Misses        444      444           

Impacted Files	Coverage Δ
seaborn/_core/plot.py	95.13% <100.00%> (+0.02%)	⬆️
seaborn/_marks/base.py	96.72% <100.00%> (ø)
seaborn/tests/_core/test_plot.py	98.38% <100.00%> (+0.01%)	⬆️

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