No, this should be `extend`. Think of plots as lists of lines or surfaces (i.e. Series). Python's lists already have append and extend methods, we just want to have the same methods here.

Your fix is perfect, just remove the `elif` block and leave your new if/else block.

Also, in the docstring you might want to use doctests so that it renders nicely in sphinx or other docviewers. Your current formatting will render as simple text. Check other docstrings in the module for examples.