Dates

[tbreloff]

Basic support for passing in arrays of dates (and datetimes) for the x-axis. Some backends won't support it, so have a standard way to gracefully not error... probably replace the date with an integer, and datetimes with a float, representing the number of days since epoch.

[tbreloff]

For anyone that looks at this... certain backends (for example, Plotly) will support dates. I'd like to add a datesSupported() method to automate checks/warnings on date inputs by backend.

[tbreloff]

Dates are supported in the pyplot backend through the discrete_values mechanic, and should work in other backends once that's properly supported. I added a checkbox to those meta-issues, so closing this.