Add support for plotting subclasses of cftime.datetime

[spencerkclark]

We would love to use nc-time-axis in xarray. There was some discussion over the summer about possibly adding support for plotting subclasses of cftime.datetime (e.g. cftime.DatetimeNoLeap) directly. This would make using it in xarray somewhat easier. This PR is an attempt to add that. In so doing, I've retained support for plotting nc_time_axis.CalendarDateTime objects, so this should be backwards-compatible.

Let me know if there is still interest in this. I'm happy to iterate on it if desired.

For example, with this PR one can now do something like this:

import random

import matplotlib.pyplot as plt
import nc_time_axis
import cftime

d_time = [cftime.Datetime360Day(year=2017, month=2, day=n) for n in range(1, 31)]
temperatures = [round(random.uniform(0, 12), 3) for _ in range(len(d_time))]

plt.plot(d_time, temperatures)
plt.margins(0.1)
plt.ylim(0, 12)
plt.xlabel("Date")
plt.ylabel("Temperature")
plt.show()

[coveralls]

Coverage increased (+0.7%) to 91.228% when pulling 0ab0395 on spencerkclark:raw-cftime into 46aee1a on SciTools:master.

[jhamman]

ping @pelson and @DPeterK for a review.

[spencerkclark]

@lbdreyer when you get a chance please let me know if you would be interested in this. Thanks!

If this seems like something you would rather wait to think about, we'll probably go forward with using CalendarDateTime in xarray for now (pydata/xarray#2665, cc: @jbusecke).

[lbdreyer]

Thanks for this @spencerkclark !

This looks like a nice clean addition that's ready for merging.

There is one more thing we need from you before I can merge this. Could you fill in this CLA form https://scitools.org.uk/cla/v4/form?

Once that's done we can merge this in and then we can cut a new release of nc-time-axis so that you are unblocked!

[spencerkclark]

Once that's done we can merge this in and then we can cut a new release of nc-time-axis so that you are unblocked!

Awesome, @lbdreyer, thank you! I just signed the CLA. I really appreciate your work on this package.

[lbdreyer]

I just signed the CLA

Thank you!