ylims broken for evoked.plot(gfp='only')

[jona-sassenhagen]

Description of the problem

With evoked.plot(gfp='only'), the ylim upper bound is not set properly, but to 1.

Steps to reproduce

Run this tutorial:

https://mne.tools/stable/auto_tutorials/evoked/30_eeg_erp.html#sphx-glr-auto-tutorials-evoked-30-eeg-erp-py

`l_aud.plot(gfp="only")`

Link to data

No response

Expected results

Something like

Actual results

Additional information

[drammock]

argh, regressions like this make me seriously consider image-diff regression tests for our tutorials. It's a big can of worms but Matplotlib does it, maybe we can borrow some best practices from them.

[jona-sassenhagen]

@drammock did you see #13266 ? :)

[drammock]

@drammock did you see #13266 ? :)

indeed I did