TST: Address UserWarning in matplotlib test
e.g. https://github.com/pandas-dev/pandas/actions/runs/9759909211/job/26937595679?pr=59136

closes #58851
I think this issue is related:  https://github.com/pandas-dev/pandas/issues/58851 Thanks @Dr-Irv! Yeah It appears something changed on the matplotlib side to make "automatic" legend labeling now raise a UserWarning  Going to merge to get CI to green