Legend tuple handler improve

This needs a bit more attention, but useful PR (I came up with a much-less clean way of doing [the same thing](https://stackoverflow.com/questions/18007022/two-unique-marker-symbols-for-one-legend/18007565#18007565))

  - needs better doc
  - needs maybe better kwarg name

replaces and closes #2034
Needs a docstring to describe these new arguments.