Keep overall trendline off marginal subplots

[jayzhou2309]

Link to issue

Closes #4669

Description of change

With trendline_scope="overall", make_figure added the overall trendline with row="all", col="all", exclude_empty_subplots=True. The marginal_x row and marginal_y column already hold the box/histogram/violin traces, so they are not empty, and the trendline was drawn on them too. This PR adds the trendline only to the main cells: every row except the top one when marginal_x is set, and every column except the right one when marginal_y is set. Facets keep one trendline per facet as before.

Demo

px.scatter(px.data.tips(), x="total_bill", y="tip", color="sex", marginal_x="box", marginal_y="box", trendline="ols", trendline_scope="overall"), axes of the Overall Trendline traces:

• main (d586d22): (x, y), (x2, y2), (x3, y3)
• this PR: (x, y)

Testing strategy

• test_overall_trendline_skips_marginals checks that the overall trendlines sit on exactly the subplots that hold the scatter points, for marginal_x + marginal_y, marginal_x + facet_col, and marginal_y + facet_row, across the pandas, pyarrow and polars backends. It fails on main (9 failed) and passes here (9 passed).
• tests/test_optional/test_px and tests/test_core: 1697 passed, 11 skipped, 4 xfailed.
• Checked by hand that facet_col_wrap=2 with marginal_x gives one trendline per facet, none on the marginal row, and one legend entry.
• ruff check and ruff format --check pass on the changed files.

AI disclosure

An AI agent (Claude Code) reproduced the issue, wrote the failing test and the fix, and ran the commands above.

Guidelines

• I have reviewed the pull request guidelines and the Code of Conduct and confirm that this PR follows them.
• I have added an entry to the changelog if needed (not required for documentation PRs).

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_{🤖 Written and posted by an AI agent (Claude Code) on behalf of @jayzhou2309.}