PR Summary

resolves #25119.

I'm new to Matplotlib development so not sure if this is the best way to do this. I added a transform parameter to secondary_xaxis and secondary_yaxis and passed that through into SecondaryAxis._set_location(), where the transform is blended.

if self._orientation == 'x':
    # An x-secondary axes is like an inset axes from x = 0 to x = 1 and
    # from y = pos to y = pos + eps, in the parent's transAxes coords.
    bounds = [0, self._pos, 1., 1e-10]

    # If a transformation is provided, use its y component rather than
    # the parent's transAxes. This can be used to place axes in the data
    # coords, for instance.
    if transform is not None:
        transform = transforms.blended_transform_factory(
            self._parent.transAxes, transform)
else:  # 'y'
    bounds = [self._pos, 0, 1e-10, 1]
    if transform is not None:
        transform = transforms.blended_transform_factory(
            transform, self._parent.transAxes)  # Use provided x axis

# If no transform is provided, use the parent's transAxes
if transform is None:
    transform = self._parent.transAxes

I also updated test_secondary_xy() and test_secondary_fail() to test the transform.

I'm not entirely sure if/where I'm meant to put .. versionchanged:: in the docsting, but I'm happy to add it if it's needed.

PR Checklist

Documentation and Tests

• Has pytest style unit tests (and pytest passes)
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• New plotting related features are documented with examples.

Release Notes

• New features are marked with a .. versionadded:: directive in the docstring and documented in doc/users/next_whats_new/
  • API changes are marked with a .. versionchanged:: directive in the docstring and documented in doc/api/next_api_changes/
• Release notes conform with instructions in next_whats_new/README.rst or next_api_changes/README.rst