Colorbar labels are not appearing when plotted in subplots as cartopy projections

Question

The colorbar appears nicely, but no values come with it. See the code below.

The code plots 2 columns, each with 4 rows for the maps and an additional row for the colorbars. The labels on the colorbars don't appear, and I don't know why.

import numpy as np
import matplotlib.pyplot as plt
import cartopy.crs as ccrs
from cartopy.mpl.ticker import LatitudeFormatter, LongitudeFormatter

# for labeling lat and lon
lon_formatter = LongitudeFormatter(zero_direction_label=True)
lat_formatter = LatitudeFormatter()

# lon and lat
lon = np.linspace(-180,180,360)
lat = np.linspace(0,90,90)

# initate figure
fig, ax = plt.subplots(5,2,subplot_kw={'projection':  ccrs.PlateCarree(central_longitude=180)}, \
                       gridspec_kw={'height_ratios': [0.33,0.33,0.33,0.33,0.05],'width_ratios': [1, 1], 'wspace': 0.1, 'hspace': 0.1},figsize=[15,10])

# mundhenk plotting
for i in range(0,4):

    # create map
    referenced_data = 20.0 * np.random.rand(90,360)
    im1 = ax[i,0].contourf(lon,lat,referenced_data,cmap='Blues',levels=np.linspace(0,20,6),extend='max')
    ax[i,0].coastlines()
    ax[i,0].set_xticks([0, 60, 120, 180, 240, 300, 360], crs=ccrs.PlateCarree())
    ax[i,0].set_yticks([0, 20, 40, 60, 80], crs=ccrs.PlateCarree())
    if i < 3:
        ax[i,0].set_xticklabels([])
        ax[i,0].yaxis.set_major_formatter(lat_formatter)
    else:
        ax[i,0].xaxis.set_major_formatter(lon_formatter)
        ax[i,0].yaxis.set_major_formatter(lat_formatter)
    ax[i,0].set_ylim(20,80)
    ax[i,0].set_xlim(-100,150)

# Add a colorbar for all subplots
cbar1 = plt.colorbar(im1, cax=ax[4,0], orientation='horizontal', label='(AR Day) / day')
cbar1.ax.set_aspect(0.075)

# tempestextreme plotting
for i in range(0,4):

    # create map
    referenced_data = 20.0 * np.random.rand(90,360)
    im2 = ax[i,1].contourf(lon,lat,referenced_data,cmap='Blues',levels=np.linspace(0,20,6),extend='max')
    ax[i,1].coastlines()
    ax[i,1].set_xticks([0, 60, 120, 180, 240, 300, 360], crs=ccrs.PlateCarree())
    ax[i,1].set_yticks([0, 20, 40, 60, 80], crs=ccrs.PlateCarree())
    if i < 3:
        ax[i,1].set_xticklabels([])
    else:
        ax[i,1].xaxis.set_major_formatter(lon_formatter)
    ax[i,1].set_yticklabels([])
    ax[i,1].set_ylim(20,80)
    ax[i,1].set_xlim(-100,150)

cbar2 = plt.colorbar(im2,cax=ax[4,1],orientation='horizontal')
cbar2.ax.set_aspect(0.075)

Answer 1

• ax[4, 0] and ax[4, 1] are specified as ccrs.PlateCarree projections, which is causing the colorbar to display incorrectly.
  • Similar to How to combine 3d projections with 2d subplots and set the width, certain objects may not work with a projection, but it's possible to combine different projections, or None, in subplots.
• The easiest option seems to be:
  1. Create the subplots as you are doing with gridspec, so a position will be allocated for the colorbars.
  2. Remove the two Axes allocated for the colorbars.
  3. Add new subplots, in the same position, that aren't projections.
  • It's easiest because it doesn't require changing the existing loops.

import numpy as np
import matplotlib.pyplot as plt
import cartopy.crs as ccrs
from cartopy.mpl.ticker import LatitudeFormatter, LongitudeFormatter

# for labeling lat and lon
lon_formatter = LongitudeFormatter(zero_direction_label=True)
lat_formatter = LatitudeFormatter()

# lon and lat
lon = np.linspace(-180, 180, 360)
lat = np.linspace(0, 90, 90)

# initate figure
fig, ax = plt.subplots(5, 2, subplot_kw={'projection': ccrs.PlateCarree(central_longitude=180)},
                       gridspec_kw={'height_ratios': [0.33, 0.33, 0.33, 0.33, 0.05],
                                    'width_ratios': [1, 1],
                                    'wspace': 0.1, 'hspace': 0.1},
                       figsize=[15, 10])

# remove the Axes with a projection
ax[4, 0].remove()
ax[4, 1].remove()

# add new subplots that aren't projections
ax9 = fig.add_subplot(5, 2, 9)
ax10 = fig.add_subplot(5, 2, 10)

# mundhenk plotting
for i in range(0, 4):

    # create map
    referenced_data = 20.0 * np.random.rand(90,360)
    im1 = ax[i,0].contourf(lon,lat,referenced_data,cmap='Blues',levels=np.linspace(0,20,6),extend='max')
    ax[i,0].coastlines()
    ax[i,0].set_xticks([0, 60, 120, 180, 240, 300, 360], crs=ccrs.PlateCarree())
    ax[i,0].set_yticks([0, 20, 40, 60, 80], crs=ccrs.PlateCarree())
    if i < 3:
        ax[i,0].set_xticklabels([])
        ax[i,0].yaxis.set_major_formatter(lat_formatter)
    else:
        ax[i,0].xaxis.set_major_formatter(lon_formatter)
        ax[i,0].yaxis.set_major_formatter(lat_formatter)
    ax[i,0].set_ylim(20,80)
    ax[i,0].set_xlim(-100,150)

# Add a colorbar for all subplots
cbar1 = fig.colorbar(im1, cax=ax9, orientation='horizontal', label='(AR Day) / day')
cbar1.ax.set_aspect(0.075)

# tempestextreme plotting
for i in range(0, 4):

    # create map
    referenced_data = 20.0 * np.random.rand(90,360)
    im2 = ax[i,1].contourf(lon,lat,referenced_data,cmap='Blues',levels=np.linspace(0,20,6),extend='max')
    ax[i,1].coastlines()
    ax[i,1].set_xticks([0, 60, 120, 180, 240, 300, 360], crs=ccrs.PlateCarree())
    ax[i,1].set_yticks([0, 20, 40, 60, 80], crs=ccrs.PlateCarree())
    if i < 3:
        ax[i,1].set_xticklabels([])
    else:
        ax[i,1].xaxis.set_major_formatter(lon_formatter)
    ax[i,1].set_yticklabels([])
    ax[i,1].set_ylim(20,80)
    ax[i,1].set_xlim(-100,150)

cbar2 = fig.colorbar(im2, cax=ax10, orientation='horizontal')
cbar2.ax.set_aspect(0.075)

---

• As pointed out by RuthC, matplotlib.pyplot.subplot_mosaic may be used to create per-Axes subplot keyword arguments.
  • Note the differences in:
    1. Creating the Axes object
    2. How the Axes objects are referenced, and iterated through

# for labeling lat and lon
lon_formatter = LongitudeFormatter(zero_direction_label=True)
lat_formatter = LatitudeFormatter()

# lon and lat
lon = np.linspace(-180, 180, 360)
lat = np.linspace(0, 90, 90)

# create subplot mosaic with different keyword arguments
fig, ax = plt.subplot_mosaic("AB;CD;EF;GH;IJ",
                             per_subplot_kw={('A', 'B', 'C', 'D', 'E', 'F', 'G', 'H'): {'projection': ccrs.PlateCarree(central_longitude=180)}},
                             gridspec_kw={'height_ratios': [0.33, 0.33, 0.33, 0.33, 0.05],
                                          'width_ratios': [1, 1],
                                          'wspace': 0.1, 'hspace': 0.1},
                             figsize=[15, 10])

# mundhenk plotting; iterate through the left Axes letters
for v, i in enumerate(['A', 'C', 'E', 'G']):

    # create map
    referenced_data = 20.0 * np.random.rand(90,360)
    im1 = ax[i].contourf(lon,lat,referenced_data,cmap='Blues',levels=np.linspace(0,20,6),extend='max')
    ax[i].coastlines()
    ax[i].set_xticks([0, 60, 120, 180, 240, 300, 360], crs=ccrs.PlateCarree())
    ax[i].set_yticks([0, 20, 40, 60, 80], crs=ccrs.PlateCarree())
    if v < 3:
        ax[i].set_xticklabels([])
        ax[i].yaxis.set_major_formatter(lat_formatter)
    else:
        ax[i].xaxis.set_major_formatter(lon_formatter)
        ax[i].yaxis.set_major_formatter(lat_formatter)
    ax[i].set_ylim(20,80)
    ax[i].set_xlim(-100,150)

# Add a colorbar for all subplots
cbar1 = fig.colorbar(im1, cax=ax['I'], orientation='horizontal', label='(AR Day) / day')
cbar1.ax.set_aspect(0.075)

# tempestextreme plotting; iterate through the right Axes letters
for v, i in enumerate(['B', 'D', 'F', 'H']):

    # create map
    referenced_data = 20.0 * np.random.rand(90,360)
    im2 = ax[i].contourf(lon,lat,referenced_data,cmap='Blues',levels=np.linspace(0,20,6),extend='max')
    ax[i].coastlines()
    ax[i].set_xticks([0, 60, 120, 180, 240, 300, 360], crs=ccrs.PlateCarree())
    ax[i].set_yticks([0, 20, 40, 60, 80], crs=ccrs.PlateCarree())
    if v < 3:
        ax[i].set_xticklabels([])
    else:
        ax[i].xaxis.set_major_formatter(lon_formatter)
    ax[i].set_yticklabels([])
    ax[i].set_ylim(20,80)
    ax[i].set_xlim(-100,150)

cbar2 = fig.colorbar(im2, cax=ax['J'], orientation='horizontal')
cbar2.ax.set_aspect(0.075)