datetime locator doesn't work with cftime

Code sample, a copy-pastable example if possible

import proplot as plot
import numpy as np
import xarray as xr

times = xr.cftime_range('1990', '2000', freq='M')
data = xr.DataArray(np.random.rand(len(times)),
    dims=['time'],
    coords=[times])

f, ax = plot.subplots(aspect=2)

ax.plot(data.time, data)

# ax.format(xlocator='year')
# ax.format(xformatter='concise')

Actual result vs. expected result

The x datetime axis appears by default (with the comments included above), but when using ax.format(xlocator='year') or any locator, the xticks and labeling completely disappear.

When using any formatter (e.g. xformatter='concise') it breaks with the following error:

ValueError: Cannot convert -3652 to a date.  This often happens if non-datetime values are passed to an axis that expects datetime objects.

[bradyrx]

I’m reopening this based on our discussion at https://github.com/SciTools/nc-time-axis/issues/41. It looks like nc-time-axis is sort of a dead repo. This is on the longer-term list for proplot developments.

[lukelbd]

After the reorganization, the cftime additions should go in ticker.py. This isn’t a priority for me right now, but this is an ideal issue for collaboration if someone else wants to try