Make pyplot more tolerant wrt. 3rd-party subclasses.

Don't force them to reproduce the names of "intended-as-positional"
arguments; don't force them to support the data kwarg.

This is also necessary for methods that take `*args`, as otherwise
we generate a call of the form `foo=foo, *args` which is incorrectly
understood as `*args, foo=foo`.

Author: anntzer