matplotlib: shorten a colorbar by half when the colorbar is created using axes_grid1

Question

I am trying to shorten a colorbar by half. Does anyone know how to do this? I tried cax.get_position() and then cax.set_position(), but this method did not work.

Besides, it seems that axes created by axes_grid1 has the same bbox positions as the original axes. Is this a bug?

PS. I have to use axes_grid1 to create colorbar axes, because I need to use tight_layout() afterwards, and tight_layout() only applies to axes created by axes_grid1 but not ones created by add_axes().

import matplotlib.pyplot as plt
from mpl_toolkits.axes_grid1 import make_axes_locatable
import numpy as np
plt.figure()
ax = plt.gca()
im = ax.imshow(np.arange(100).reshape((10,10)))
divider = make_axes_locatable(ax)
cax = divider.append_axes("right", size="5%", pad=0.05)
bbox1 = ax.get_position()
print(bbox1)
bbox1 = cax.get_position()
print(bbox1)
plt.colorbar(im, cax=cax)
plt.show()

Answer 1

The whole point of the axes_divider is to divide the axes to make space for a new axes. This ensures that all axes have the same surrounding box. And that is the box you see being printed.

Some of the usual ways to create a colorbar, at a certain location in the figue are shown in this question. Here the problem seems to be to be able to call tight_layout. This is achievable with the following two options. (There might be others still.)

A. using gridspec

I'm not too sure about the exact requirements here, but it seems that using a normal grid layout would be more in the direction of what you need here.

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.gridspec as gridspec

fig = plt.figure()
gs = gridspec.GridSpec(2, 2, width_ratios=[95,5],)

ax = fig.add_subplot(gs[:, 0])

im = ax.imshow(np.arange(100).reshape((10,10)))

cax = fig.add_subplot(gs[1, 1])
fig.colorbar(im, cax=cax, ax=ax)
plt.tight_layout()
plt.show()

B. Using axes_grid1

If you really need to use axes_grid1, it might become a little bit more complicated.

import matplotlib.pyplot as plt
import matplotlib.axes
from mpl_toolkits.axes_grid1 import make_axes_locatable, Size
import numpy as np

fig, ax = plt.subplots()

im = ax.imshow(np.arange(100).reshape((10,10)))

divider = make_axes_locatable(ax)

pad = 0.03
pad_size = Size.Fraction(pad, Size.AxesY(ax))

xsize = Size.Fraction(0.05, Size.AxesX(ax))
ysize = Size.Fraction(0.5-pad/2., Size.AxesY(ax))

divider.set_horizontal([Size.AxesX(ax), pad_size, xsize])
divider.set_vertical([ysize, pad_size, ysize])

ax.set_axes_locator(divider.new_locator(0, 0, ny1=-1))


cax = matplotlib.axes.Axes(ax.get_figure(),
                 ax.get_position(original=True))
locator = divider.new_locator(nx=2, ny=0)
cax.set_axes_locator(locator)

fig.add_axes(cax)

fig.colorbar(im, cax=cax)

plt.tight_layout()
plt.show()