fix(visualization): keep layer arrow in frame for linear flows

[shantoshdurai]

Fixes #535

Linear open graphs place every node on one row, so ymin == ymax. Layer labels use fixed point offsets below plot_lims.ymin, but the ymin adjustment in _draw_layers ran before axis limits were set, so the data/display transform was degenerate and the "Layer" arrow ended up below the plot frame.

Changes

• Set preliminary plot limits before _draw_layers so point-based offsets map correctly
• Expand ymin to include layer label height (va='top') plus padding
• Enforce a minimum vertical axis span in _set_plot_lims when node rows collapse to one line
• Add regression test test_linear_causal_flow_layer_label_in_frame

Verify

pytest tests/test_visualization.py::test_linear_causal_flow_layer_label_in_frame -q

[codecov]

Codecov Report

❌ Patch coverage is 78.57143% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.83%. Comparing base (af1c16d) to head (03302b3).

Files with missing lines	Patch %	Lines
graphix/visualization.py	78.57%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #536      +/-   ##
==========================================
- Coverage   88.85%   88.83%   -0.03%     
==========================================
  Files          49       49              
  Lines        7135     7146      +11     
==========================================
+ Hits         6340     6348       +8     
- Misses        795      798       +3     

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[matulni]

Hi @shantoshdurai, thanks for your contribution!
Our policy is that CI should pass before engaging in a PR review. Don't hesitate to ask if you have any questions.

In addition, to clear up any possible confusion, note that this issue is not part of UnitaryHack 2026, and there is no bounty on it.

[thierry-martinez]

The CI is broken because of a new Matplotlib release. It was fixed in  #541, which has now been merged. Could you merge the current master into your branch?