Not converting values when passed a list

[aulemahal]

It appears that some matplotlib method pass lists of coordinates to the converting methods. Such an example is plt.axhspan, that passes [xmin, xmax] to ax.convert_xunits and, thus, down to nc_time_axis.NetCDFTimeConverter.convert. However, the latter tests for np.ndarray but not for lists, so it raises a ValueError.

MWE:

import cftime
import matplotlib.pyplot as plt

x1 = cftime.DatetimeNoLeap(1990, 1, 1)
x2 = cftime.DatetimeNoLeap(1991, 1, 1)
x3 = cftime.DatetimeNoLeap(1992, 1, 1)

plt.plot([x1, x2, x3], [1, 2, 1])
plt.axvspan(x1, x2, color='red', alpha=0.5)

Expected:

Result:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
~/.conda/envs/xclim-dev/lib/python3.8/site-packages/matplotlib/axis.py in convert_units(self, x)
   1519         try:
-> 1520             ret = self.converter.convert(x, self.units, self)
   1521         except Exception as e:

~/.conda/envs/xclim-dev/lib/python3.8/site-packages/nc_time_axis/__init__.py in convert(cls, value, unit, axis)
    277         if not isinstance(first_value, (CalendarDateTime, cftime.datetime)):
--> 278             raise ValueError('The values must be numbers or instances of '
    279                              '"nc_time_axis.CalendarDateTime" or '

ValueError: The values must be numbers or instances of "nc_time_axis.CalendarDateTime" or "cftime.datetime".

The above exception was the direct cause of the following exception:

ConversionError                           Traceback (most recent call last)
<ipython-input-47-fe6cec84569d> in <module>
      4 
      5 plt.plot([x1, x2, x3], [1, 2, 1])
----> 6 plt.axvspan(x1, x2, color='red', alpha=0.5)

~/.conda/envs/xclim-dev/lib/python3.8/site-packages/matplotlib/pyplot.py in axvspan(xmin, xmax, ymin, ymax, **kwargs)
   2460 @_copy_docstring_and_deprecators(Axes.axvspan)
   2461 def axvspan(xmin, xmax, ymin=0, ymax=1, **kwargs):
-> 2462     return gca().axvspan(xmin, xmax, ymin=ymin, ymax=ymax, **kwargs)
   2463 
   2464 

~/.conda/envs/xclim-dev/lib/python3.8/site-packages/matplotlib/axes/_axes.py in axvspan(self, xmin, xmax, ymin, ymax, **kwargs)
   1105 
   1106         # first we need to strip away the units
-> 1107         xmin, xmax = self.convert_xunits([xmin, xmax])
   1108         ymin, ymax = self.convert_yunits([ymin, ymax])
   1109 

~/.conda/envs/xclim-dev/lib/python3.8/site-packages/matplotlib/artist.py in convert_xunits(self, x)
    173         if ax is None or ax.xaxis is None:
    174             return x
--> 175         return ax.xaxis.convert_units(x)
    176 
    177     def convert_yunits(self, y):

~/.conda/envs/xclim-dev/lib/python3.8/site-packages/matplotlib/axis.py in convert_units(self, x)
   1520             ret = self.converter.convert(x, self.units, self)
   1521         except Exception as e:
-> 1522             raise munits.ConversionError('Failed to convert value(s) to axis '
   1523                                          f'units: {x!r}') from e
   1524         return ret

ConversionError: Failed to convert value(s) to axis units: [cftime.DatetimeNoLeap(1990, 1, 1, 0, 0, 0, 0), cftime.DatetimeNoLeap(1991, 1, 1, 0, 0, 0, 0)]

I hacked this into nc-time-axis (NetCDFTimeConverter.convert):

if isinstance(value, (list, tuple)):
    first_value = value

to make it work. However, I am not an expert of matplotlib and I have no idea if this could be dangerous or if it is appropriate...

[5tefan]

I'm also running into this issue with ax.axvspan. My (hopefully temporary) workaround is to call nc_time_axis.NetCDFTimeConverter().convert(bounds, None, None) where bounds contains the two DatetimeGregorian objects for something like:

#...
ax.axvspan(
    *nc_time_axis.NetCDFTimeConverter().convert(bounds, None, None),
    color="red",
    alpha=0.15
)
#...

[bjlittle]

@aulemahal Is this still a live issue that we want to pull into v1.3.0 ?

[aulemahal]

Yes it is still a live issue! However, I see no rush to put it in v1.3.0. I'll try to open a PR with a solution, and if it is done in time, we'll have it there!