feat: add plot_ts for time series visualization (closes #322)

[jinukuntlaakhilakumargoud-web]

Summary

Implements plot_ts - a new time series plot (issue #322). This adds parity with the legacy arviz.plot_ts function using the new modular PlotCollection-based architecture, following the pattern established by plot_lm.

New file: src/arviz_plots/plots/ts_plot.py

Visual elements

Key	Visual fn	Description
observed_line	line_xy	Observed data line (pre-holdout)
posterior_predictive	line_xy	N posterior predictive sample lines
observed_scatter	scatter_xy	Holdout observed data points
forecast	line_xy	N forecast sample lines (post-split)
vline	vline	Vertical divider at train/holdout boundary
xlabel / ylabel	labelled_x/y	Axis labels

Key parameters

• y, x - pre-holdout observed variable and time axis
• • y_hat - posterior predictive variable for in-sample trajectories
• • y_holdout, y_forecasts - holdout observed and forecast variables
• • num_samples - number of PP sample lines to draw (default 50)
• • sample_dims, coords, filter_vars, visuals, aes_by_visuals, **pc_kwargs - standard ArviZ-plots pattern

Implementation notes

• Follows the ppc_dist_plot pattern for PP samples: stacks sample_dims into a flat sample dimension and uses overlay_ppc aesthetic
• • Uses a _combine_xy helper (similar to combine_sort_smooth in plot_lm) to build plot_axis-shaped datasets for line_xy / scatter_xy
• • Correctly renames holdout/forecast variables to match y variable names for PlotCollection.map() alignment

Tests: tests/test_ts_plot.py

7 tests covering:

• Basic observed line only
• • Posterior predictive sample lines (y_hat)
• • Holdout scatter + vline (y_holdout)
• • Forecast lines (y_forecasts)
• • Full combination (all visuals)
• • Visual disabling (visuals={"observed_line": False})
• • Error handling (invalid plot_dim)

pytest tests/test_ts_plot.py -k none
# 7 passed

[read-the-docs-community]

Documentation build overview

📚 arviz-plots | 🛠️ Build #31586926 | 📁 Comparing 63cf15a against latest (294b5c5)

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Show files changed (57 files in total): 📝 57 modified | ➕ 0 added | ➖ 0 deleted

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api/generated/arviz_plots.PlotCollection.generate_aes_dt.html	📝 modified

[aloctavodia]

@jinukuntlaakhilakumargoud-web, usually, if you see that another person says they are working on an issue, you should not work on that issue.

[aloctavodia]

@rohanbabbar04 can you check this?

[jinukuntlaakhilakumargoud-web]

@aloctavodia Ah, I sincerely apologize! I am still getting used to the open-source workflow and completely missed that @rohanbabbar04 had already claimed issue #322 before I started. That is entirely my fault.

@rohanbabbar04 I didn't mean to step on your toes. Since I already pushed this up, please feel completely free to use my branch as a starting point, review this PR, or if you prefer to build it from scratch yourself, just let me know and I will close this PR! Whatever works best for you.

[OriolAbril]

@jinukuntlaakhilakumargoud-web for future reference: https://python.arviz.org/en/latest/contributing/contributing_prs.html#steps-before-starting-work (most relevant piece quoted below)

If an issue ticket exists

If an issue exists, check the ticket to ensure no one else has started working on it. If you are first to start work, comment on the ticket to make it evident to others. If the comment looks old or abandoned, leave a comment asking if you may start work.

[rohanbabbar04]

@rohanbabbar04 can you check this?

Yeah, I can check this