Adding an option to restrict the limits of the gridlines.

[greglucas]

Rationale

Polar projections often look better without gridlines going all the way to the poles.

Modifying the example from 1568:

import cartopy
import cartopy.crs as ccrs
import numpy as np
import matplotlib.pyplot as plt
import cartopy.feature as cfeature
import matplotlib.ticker as mticker
lats = mticker.FixedLocator(np.arange(-80, 81, 5))
lons = mticker.MultipleLocator(60)
fig = plt.figure(figsize=(6, 3))

ax = fig.add_subplot(121, projection=ccrs.NorthPolarStereo())
ax.set_extent([-180, 180, 70, 90], crs=ccrs.PlateCarree())
ax.gridlines(xlocs=lons, ylocs=lats, ylim=80, xlim=120)
ax.add_feature(cfeature.COASTLINE)

ax = fig.add_subplot(122, projection=ccrs.SouthPolarStereo())
ax.set_extent([-180, 180, -90, -70], crs=ccrs.PlateCarree())
ax.gridlines(xlocs=lons, ylocs=lats, ylim=80, xlim=120)
ax.add_feature(cfeature.COASTLINE)

fig.suptitle(f'Gridlines over polar projections in cartopy v{cartopy.__version__}')

Implications

Fixes #1568

[lukelbd]

Thanks for submitting this! I'm not sure there's even a use case for latitude gridlines that do not extend through all longitudes, so I'm not sure if xlim is even necessary (though that's just from my narrow experience, perhaps others would disagree). It would also be complicated to implement this in cases where central_longitude != 0 -- the gridliner has some existing problems when this is the case (e.g. #1401, #1564).

And maybe something like ymax (similar to latmax in basemap) would make more sense -- to me ylim implies a (ymin, ymax) pair like when you use ax.set_ylim(ylim) on Cartesian axes. If the keyword args xlim and ylim are used they should probably be (minimum, maximum) pairs.

[lukelbd]

Scratch that last comment, just looked at the source code. Looks good to me!

[dopplershift]

Looks good to me!