Add support for barcharts with x-axis date data.
Assume utc time for all dates. This is strictly correct (see below).

Matplotlib's docs state:
'objects are converted to floating point numbers which represent time
in days since 0001-01-01 UTC, plus 1.'

We convert to epoch time with mpl, so it's safe to assume we've got
utc time!

http://matplotlib.org/api/dates_api.html#module-matplotlib.dates