Is marker guaranteed to be a string? Because in Matplotlib itself (not necessarily the Qt editor), it could be an array_like, which would cause a warning when comparing to a string.

I've tested this and marker is a matplotlib marker definition, i.e. a plain python str or int (https://matplotlib.org/gallery/lines_bars_and_markers/marker_reference.html). So it's ok.

Markers are also allowed to be Paths, as in https://matplotlib.org/gallery/shapes_and_collections/marker_path.html or NumPy arrays, but both of those seem to be broken in some other ways than this PR.