pandas datetimeindex raises no error/warnings

[kuzha]

I used funnel in a test, where pandas dataframes needed to be compared. The x axis inputs (reference and test) are pandas datetimeindex. pyfunnel did not raise any error, instead it converted the datetimeindex into some unknown floating numbers.

Converting datetimeindex into epoch time solved this problem. Perhaps this conversion could be handled by funnel internally, or raise a warning at least.