It'd be nice to be able to register artists with the legend on the fly rather than on creation (plt.plot(label='my-label')) or all at once (plt.legend([h1, h1], ['label1, label2'])). I think this would be helpful for managing multiple legends or to modify a third party legend.

You basically have to add a method Legend.add_entry(handle, label) that properly updates the internal state.

Originally posted by @timhoffm in #29422 (comment)