[ar1_turningpts] arviz 1.x: az.plot_trace silently drops the density panels — plot_trace_dist is the 0.x equivalent

Summary

Under arviz 1.x, az.plot_trace and az.plot_trace_dist are two different plots, and the name that carries over from arviz 0.x is the wrong one. lectures/ar1_turningpts.md was correctly migrated to plot_trace_dist in #584, but PR #1020 changes it back to plot_trace, which silently drops the marginal density panels from the figure. lectures/ar1_bayes.md has the same problem, left over from the #930 build fix, and was never migrated.

Neither case fails the build — the figure just quietly loses half its content.

What changed in arviz 1.x

Straight from the 1.2.0 docstrings:

So a lecture that used az.plot_trace under 0.x and still calls az.plot_trace under 1.x renders a strictly smaller figure than it used to, with no warning and no error. plot_trace_dist is the call that preserves the original output.

Affected lectures

How this happened

The three commits tell the story:

• 2026-06-25, #930 — dropped the figsize kwarg from az.plot_trace in ar1_turningpts.md to unbreak the build under arviz 1.x. A minimal fix that left the call as trace-only.
• 2026-07-16, #584 — deliberately upgraded ar1_turningpts.md to az.plot_trace_dist(plot_data, var_names=['ρ', 'σ']), restoring the density panels. ar1_bayes.md was not touched.
• 2026-07-31, #1020 — reverts ar1_turningpts.md back to az.plot_trace, undoing #584. Most likely a 0.x-from-memory correction, since plot_trace is the name everyone knows.

Suggested fix

Standardise on az.plot_trace_dist in both lectures: revert the ar1_turningpts.md line in #1020 back to plot_trace_dist, and update the two ar1_bayes.md call sites to match. Both lectures install arviz unpinned at runtime, so neither should pin a version — fix forward to the 1.x API, consistent with how the other in-lecture pip packages are handled.

Worth a short comment at the call site noting that plot_trace_dist is the 1.x name for what used to be plot_trace, so the next person reading it doesn't "fix" it again.

Related

Same arviz 0.x → 1.x class of breakage as the az.from_dict and az.summary failures in sargent_surico.md, diagnosed in QuantEcon/lecture-python.myst#1020. Earlier instances: #929, #935.

Read the affected call sites in lectures/ar1_turningpts.md and lectures/ar1_bayes.md, comparing them with the migration described in the issue. Restore az.plot_trace_dist in ar1_turningpts.md and update both ar1_bayes.md calls, then render or build the lectures and confirm the marginal density panels are present.