Add graphviz in the list of optional dependencies

[NexVeridian]

closes #12477

Add graphviz in the list of optional dependencies

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.35%. Comparing base (96e1f01) to head (1ee5a7e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15755   +/-   ##
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  Coverage   81.35%   81.35%           
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  Files        1379     1379           
  Lines      176603   176603           
  Branches     2544     2544           
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+ Hits       143677   143682    +5     
+ Misses      32443    32438    -5     
  Partials      483      483           

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[alexander-beedie]

Hi @NexVeridian; thanks for the PR, but I don't think this fixes the linked issue. The graphviz Python package doesn't actually install the underlying graphviz dot executable (rather, that package helps you to create "dot" files, but the executable still needs to be separately installed to render them; as we create the "dot" file ourselves, we don't need that package to be present).

We can probably improve the docs and/or error by more explicitly pointing to the graphviz install page; would you like to amend the PR (or submit a new one) to try that instead? 🤔