Change matplotlib colorbar to custom height

Question

I would like to set the colorbar of my plot to a custom height, not necessarily to match the size of the plot. In fact I would like the height of the colorbar PLUS the title on top of it to match the height of the figure.

With

ax3 = divider.append_axes('right', size='10%', pad=0.3)
cb = plt.colorbar(Q, cax=ax3, ticks=[0.0, 3.0, 6.0, 9.0, 12.0, 15.0], format='%.1f')

I managed to have a colorbar with the same height as the plot, which has been asked for many other times, now I would like to shrink it.

Following suggestion provided in other questions I decided to explicitly give the colorbar its own axes with add_axes, after getting the position of the last plot axes with get_position. Here is what I'm trying to do. There are no data and no colorbar in this example, just to show that I'm not getting the result I expected:

   from __future__ import unicode_literals
   import numpy as np
   from scipy.interpolate import griddata
   import matplotlib
   import matplotlib.pyplot as plt
   from matplotlib.patches import Circle
   from matplotlib.pylab import cm
   import matplotlib.colors as colors
   from mpl_toolkits.axes_grid1 import make_axes_locatable, axes_size

   matplotlib.rcParams.update({'font.size': 8})

   fig = plt.figure()
   fig.set_size_inches(6.3,6.3)

   ax1 = plt.subplot(111)
   divider = make_axes_locatable(ax1)
   ax2 = divider.append_axes('right', size='100%', pad=0.3)

   axes = [ax1, ax2]
   ltypes = ['dashed', 'solid']

   xi = np.linspace(-18.125, 18.125, 11)
   yi = np.linspace(0, 28, 9)
   xv, yv = np.meshgrid(xi, yi)

   xcOdd = 0.2
   zcOdd = 0.725
   xcEven = 0.6
   zcEven = 0.725

   maskRadius = 0.15

   for i in range(2):
       ax = axes[i]
       ax.set_xlabel('distance [m]')
       if i == 0:
           ax.set_ylabel('depth [m]')
       if i == 1:
           ax.set_yticklabels([])
       ax.invert_yaxis()
       ax.tick_params(direction='in')
       ax.set_aspect('equal')
       odd = Circle((xcOdd, zcOdd), .15, linewidth=1.2, color='k', fill=False)
       even = Circle((xcEven, zcEven), .15, linewidth=1.2, linestyle=ltypes[i], color='k', fill=False)

       vmax = 15.
       vmin =  0.
       norm = matplotlib.colors.Normalize(vmin,vmax, clip=False)

       color_map = matplotlib.colors.ListedColormap(plt.cm.Greys(np.linspace(0.25, 1, 5)), "name")

       ax.add_patch(odd)
       pad = 0.03
       width = 0.03

       pos = ax2.get_position()

       ax3 = fig.add_axes([pos.xmax + pad, pos.ymin, width, 0.7*(pos.ymax-pos.ymin) ])

       plt.savefig('prova-vect-paper-test-2.eps', format='eps')

Why is get_position returning the wrong boundingbox?

Answer 1

You need to draw the canvas before obtaining the actual position from .get_position(). This is because due to the equal aspect ratio, the axes changes size and position at draw time.

import numpy as np
import matplotlib
import matplotlib.pyplot as plt
import matplotlib.colors as colors
from mpl_toolkits.axes_grid1 import make_axes_locatable

matplotlib.rcParams.update({'font.size': 8})

fig = plt.figure()
fig.set_size_inches(6.3,6.3)

ax1 = plt.subplot(111)
divider = make_axes_locatable(ax1)
ax2 = divider.append_axes('right', size='100%', pad=0.3)

axes = [ax1, ax2]


xi = np.linspace(-18.125, 18.125, 11)
yi = np.linspace(0, 28, 9)
xv, yv = np.meshgrid(xi, yi)


for i in range(2):
    ax = axes[i]
    ax.set_xlabel('distance [m]')
    if i == 0:
        ax.set_ylabel('depth [m]')
    if i == 1:
        ax.set_yticklabels([])
    ax.invert_yaxis()
    ax.tick_params(direction='in')
    ax.set_aspect('equal')
    vmax = 15.
    vmin =  0.
    norm = colors.Normalize(vmin,vmax, clip=False)

    color_map = colors.ListedColormap(plt.cm.Greys(np.linspace(0.25, 1, 5)), "name")

    im = ax.imshow(yv, cmap=color_map, norm=norm)

pad = 0.03
width = 0.03

fig.canvas.draw()
pos = ax2.get_position()

ax3 = fig.add_axes([pos.xmax + pad, pos.ymin, width, 0.7*(pos.ymax-pos.ymin) ])
fig.colorbar(im, cax=ax3)

plt.show()