fixes plotting error in declarative with hires data

[kgoebber]

Description Of Changes

This fix to the declarative syntax is to address the issue brought up by @marcinkawka in Issue #1590 where they were experiencing issues with contouring values. Upon a deep dive into the declarative code, I believe that this issue arose due to the high resolution of the data being used and the use of a modulo in generating x-component values for plotting that were problematic.

The fix to this problem is to not take the modulo of the x values being computed when transforming points from lat/lon to projection coordinates. I don't know why this was needed initially as none of the tests fail with that removed (also removed from the barb plotting aspect as well with similar result).

I have added a new test dataset based on the data used in the issue in ensure this issue is covered.

Here is an example using the new test data that reproduces the bad behavior:

from datetime import datetime

import cartopy.crs as ccrs
import cartopy.feature as cfeature
import matplotlib.pyplot as plt
from metpy.cbook import get_test_data
import numpy as np
import xarray as xr

read_test_data = xr.open_dataset(get_test_data('CAM_test.nc', False))

x, y, _ = ccrs.PlateCarree().transform_points(ccrs.PlateCarree(),
                                              *np.meshgrid(read_test_data.longitude,
                                                           read_test_data.latitude)).T
x = x[:, 0] % 360
y = y[0, :]

fig = plt.figure(1)
ax = fig.add_subplot(111, projection=ccrs.PlateCarree())
cs = ax.contour(x, y, read_test_data.PN[0,0].values, range(0, 1400, 4), linewidths=2)
ax.add_feature(cfeature.BORDERS.with_scale('50m'))
ax.add_feature(cfeature.COASTLINE.with_scale('50m'))

plt.clabel(cs, fmt='%d')
plt.show()

To get the appropriate behavior, simply remove the % 360 on the x variable.

Checklist

• Closes Contour interpolation problem #1590
• Tests added
• Fully documented

[dopplershift]

I know we needed this when we did our initial testing for the declarative. Maybe differences in datasets that are 0->360 vs. -180-> 180? I dunno, without a test we're philosophically safe to drop it.

We'll just end up having to find a way to fix it properly when we get a bug report about this...because of course we will.