Adding gridlines using Cartopy - matplotlib

Question

I'm trying to add gridlines to a map I made using Cartopy, however, when I use the example code from the cartopy documentation, it doesn't display what I want and I can't figure out how to manipulate it to do so.
def plotMap():
proj = ccrs.Mercator(central_longitude=180, min_latitude=15,
max_latitude=55)
fig, ax = plt.subplots(subplot_kw=dict(projection=proj), figsize=(12,12))
ax.set_extent([255 ,115, 0, 60], crs=ccrs.PlateCarree())
ax.add_feature(cfeature.LAND, facecolor='0.3')
ax.add_feature(cfeature.LAKES, alpha=0.9)
ax.add_feature(cfeature.BORDERS, zorder=10)
ax.add_feature(cfeature.COASTLINE, zorder=10)
#(http://www.naturalearthdata.com/features/)
states_provinces = cfeature.NaturalEarthFeature(
category='cultural', name='admin_1_states_provinces_lines',
scale='50m', facecolor='none')
ax.add_feature(states_provinces, edgecolor='black', zorder=10)
#ax.gridlines(xlocs=grids_ma, ylocs=np.arange(-80,90,20), zorder=21,
draw_labels=True )
ax.gridlines(crs=ccrs.PlateCarree(), linewidth=2, color='black',
draw_labels=True, alpha=0.5, linestyle='--')
ax.xlabels_top = False
ax.ylabels_left = False
ax.ylabels_right=True
ax.xlines = True
ax.xlocator = mticker.FixedLocator([-160, -140, -120, 120, 140, 160, 180,])
ax.xformatter = LONGITUDE_FORMATTER
ax.yformatter = LATITUDE_FORMATTER
ax.xlabel_style = {'size': 15, 'color': 'gray'}
ax.xlabel_style = {'color': 'red', 'weight': 'bold'}
return fig, ax
I've attached a picture of the output. For reference, I only want the longitude gridlines to start at the left of my domain and end at the right side, preferably being spaced every 20 degrees. Ideally the same for latitude lines as well.
Bad gridline plot

Answer 1

Is the example you are following the one at the bottom of this page? If so, you are attempting to set attributes on the GeoAxes (ax) instance which should be set on the GridLiner (gl) instance:
import cartopy
import cartopy.crs as ccrs
import cartopy.feature as cfeature
import matplotlib.pyplot as plt
import matplotlib.ticker as mticker
from cartopy.mpl.gridliner import LONGITUDE_FORMATTER, LATITUDE_FORMATTER
def plotMap():
proj = ccrs.Mercator(central_longitude=180, min_latitude=15,
max_latitude=55)
fig, ax = plt.subplots(subplot_kw=dict(projection=proj), figsize=(12,12))
ax.set_extent([255 ,115, 0, 60], crs=ccrs.PlateCarree())
ax.add_feature(cfeature.LAND, facecolor='0.3')
ax.add_feature(cfeature.LAKES, alpha=0.9)
ax.add_feature(cfeature.BORDERS, zorder=10)
ax.add_feature(cfeature.COASTLINE, zorder=10)
states_provinces = cfeature.NaturalEarthFeature(
category='cultural', name='admin_1_states_provinces_lines',
scale='50m', facecolor='none')
ax.add_feature(states_provinces, edgecolor='black', zorder=10)
gl = ax.gridlines(crs=ccrs.PlateCarree(), linewidth=2, color='black', alpha=0.5, linestyle='--', draw_labels=True)
gl.xlabels_top = False
gl.ylabels_left = False
gl.ylabels_right=True
gl.xlines = True
gl.xlocator = mticker.FixedLocator([120, 140, 160, 180, -160, -140, -120])
gl.ylocator = mticker.FixedLocator([0, 20, 40, 60])
gl.xformatter = LONGITUDE_FORMATTER
gl.yformatter = LATITUDE_FORMATTER
gl.xlabel_style = {'color': 'red', 'weight': 'bold'}
This produces the following map. The gridliner doesn't seem to be coping with the dateline. I do not know if there is a way around this, but there is a note at the top of the above linked documentation to say that there are currently known limitations with this class, so maybe not.
An alternative is to set the various labels and their styles directly with matplotlib. Note that you have to set the ticklabels separately from the ticks, otherwise you get labels corresponding to the Mercator coordinate reference system:
import cartopy.mpl.ticker as cticker
def plotMap2():
proj = ccrs.Mercator(central_longitude=180, min_latitude=15,
max_latitude=55)
fig, ax = plt.subplots(subplot_kw=dict(projection=proj), figsize=(12,12))
ax.set_extent([255 ,115, 0, 60], crs=ccrs.PlateCarree())
ax.add_feature(cfeature.LAND, facecolor='0.3')
ax.add_feature(cfeature.LAKES, alpha=0.9)
ax.add_feature(cfeature.BORDERS, zorder=10)
ax.add_feature(cfeature.COASTLINE, zorder=10)
states_provinces = cfeature.NaturalEarthFeature(
category='cultural', name='admin_1_states_provinces_lines',
scale='50m', facecolor='none')
ax.add_feature(states_provinces, edgecolor='black', zorder=10)
ax.set_xticks([120., 140., 160., 180., -160., -140., -120.], crs=ccrs.PlateCarree())
ax.set_xticklabels([120., 140., 160., 180., -160., -140., -120.], color='red', weight='bold')
ax.set_yticks([20, 40], crs=ccrs.PlateCarree())
ax.set_yticklabels([20, 40])
ax.yaxis.tick_right()
lon_formatter = cticker.LongitudeFormatter()
lat_formatter = cticker.LatitudeFormatter()
ax.xaxis.set_major_formatter(lon_formatter)
ax.yaxis.set_major_formatter(lat_formatter)
ax.grid(linewidth=2, color='black', alpha=0.5, linestyle='--')