Ignore sharex unless subplots=True

[mdruiter]

I was trying sharex=True in df.plot(). That confused me. I think it's because sharex was only designed to work with subplots=True. It does something with subplots=False (the default) as well, except it's not clear what and it doesn't make much sense.

My suggestion would be, in the case that subplots=False, to either ignore sharex (and sharey?) altogether or warn when it is specified (or both).

[TinoDerb]

I would say that at least a warning should be thrown to the user that sharing axis should be coupled with subplots=True. Am not a very big github user, this issue can be remedied by a simple condition and warn statement.