Fix JoinSpec graph visualization

[AndreRatzenberger]

Fixes JoinSpec graph visualization issues

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.54%. Comparing base (13b24cd) to head (97a1d33).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/flock/api/graph_builder.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #371      +/-   ##
==========================================
- Coverage   83.60%   83.54%   -0.07%     
==========================================
  Files         144      144              
  Lines        9225     9225              
  Branches     1170     1170              
==========================================
- Hits         7713     7707       -6     
- Misses       1212     1220       +8     
+ Partials      300      298       -2     

Flag	Coverage Δ
backend	83.54% <0.00%> (-0.07%)	⬇️

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