PR summary

When clearing an axes (via cla() or clf()) that has a shared axis with a non-linear scale (e.g., log, logit), an incorrect warning was generated:

Attempt to set non-positive xlim on a log-scaled axis will be ignored.

Why is this change necessary?

This warning appeared even though the user's code was perfectly valid. Clearing axes with shared non-linear scales is a common operation and should not generate spurious warnings.

What problem does it solve?

The issue occurred during the axis limit propagation in the Axis._set_lim() method in axis.py. When an axes with linear scale clears and resets to default limits (0, 1), these limits automatically propagate to all shared axes. If a shared axis has a non-linear scale (like log), it rejects these limits because they contain non-positive values, triggering the warning.

What is the reasoning for this implementation?

The fix adds a targeted check in the limit propagation loop (axis.py, lines 1267-1272) to skip propagating default (0, 1) limits when:

The source axis has linear scale, AND
The receiving shared axis has a non-linear scale, AND
The limits being propagated are exactly (0, 1)
This prevents the spurious warning while preserving all other propagation behavior (including for inverted shared axes, which rely on propagation to work correctly).

Additionally, in _base.py (lines 1278 and 1298), the sharex() and sharey() methods now set the scale before copying limits from the shared axis. This prevents temporary scale/limit mismatches during axis sharing initialization.

Minimal self-contained example:

import matplotlib.pyplot as plt

# Before fix: generates warning
fig, (ax1, ax2) = plt.subplots(2, 1, sharex=True)
ax1.set_xscale('log')
fig.clf()  # Warning: "Attempt to set non-positive xlim..."

# After fix: no warning

PR checklist

• "closes clearing axes with shared axis with non-linear scales should not warn #9970" is in the body of the PR description to link the related issue
• new and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• [N/A] Documentation complies with general and docstring guidelines