Mixing positional and keyword arguments for legend handles and labels...

... is no longer valid. If passing handles and labels to legend, they must now be passed either both positionally or both as keywords.

Legend labels for plot

Previously if a sequence was passed to the label parameter of ~.Axes.plot when plotting a single dataset, the sequence was automatically cast to string for the legend label. Now, if the sequence length is not one an error is raised. To keep the old behavior, cast the sequence to string before passing.