Problem

#25117 wanted to put an extra axes at y=0. It should be possible to pass ax.transData to secondary_xaxis so that the location variable is understood as data co-ordinates, not axes-relative co-ordinates.

Proposed solution

secondary_xaxis(0, transform=ax.transData) should put the secondary axes at y=0. Of course the default should be ax.transAxes for back-compatibility.