VIS: don't modify matplotlib units registry globally
Possible follow-up for https://github.com/pandas-dev/pandas/pull/18307, see comment there: https://github.com/pandas-dev/pandas/pull/18307#issuecomment-347911487 and https://github.com/pandas-dev/pandas/pull/18307#issuecomment-345637950 for the details.

Idea would be to only register our converters when we are plotting ourselves, so people plotting directly with matplotlib (`plt.plot(..)` or `ax.plot(..)` don't use our converters by default (they can still register them manually with the option).

