API: dedicated plot methods like plot.scatter as an alternative to using kind
Currently, we use the `plot` for every possible kind of plot, and switch between them using the `kind` keyword argument.

From an API design and discoverability perspective, this seems less than ideal. If we had separate methods, standard introspection tools like IPython and autocomplete could help users figure out which arguments are applicable for which type of plot with less reliance on the documentation. 

Another option is to make a `.plot` also an accessor, so we could have `df.plot.scatter` and so forth. But I suspect `.plot_scatter` would be a bit more discoverable.
