Matplotlib axis position and colorbar alignment

Question

I'm trying to align multiple colorbars with subplots generated with either gridspec or fig.add_subplots. I'd like to add the colorbar with fig.add_axes in matplotlib (v2.02), because it allows detailed alignment control. However, I need to get the figure position in order to do the alignment. The ax.get_position() function does not appear to return the correct coordinates:

Here is an MWE:

import matplotlib.gridspec as gridspec
import scipy.misc

fig = plt.figure(1)
fig.set_tight_layout(True)
gs = gridspec.GridSpec(2,2)
ax = plt.subplot(gs[0,0])
lena = scipy.misc.face()
im = ax.imshow(lena)
axpos = ax.get_position() # bbox# get_position().get_points()
cbar_axis = fig.add_axes([axpos.x0, axpos.y0-0.1, axpos.width, 0.05])
colorbar = fig.colorbar(im, cax=cbar_axis, orientation="horizontal")

The colorbar does not align:

If I print axpos after the code runs, and ax.get_position(), the two are not the same. This suggests that ax.get_position() is using stale information while the code is executing. I've tried adding plt.show() and plt.draw() and it does not fix this.

How can I get the axis position?

EDIT:

I've modified the solution posted below (https://stackoverflow.com/a/44731797/238882) to better show what I am trying to do and why make_axes_locatable does not appear (yet) to work for me. I want two colorbars under one column, and one under another column. Doing this with make_axes_locatable and the following code produces the figure below. I would like the figures to all remain equal sizes and aligned in a grid.

import matplotlib.pyplot as plt
import matplotlib.gridspec as gridspec
import scipy.misc
from mpl_toolkits.axes_grid1 import make_axes_locatable

plt.close(1)
fig, axes = plt.subplots(2,2)
fig.set_tight_layout(True)

lena = scipy.misc.face()

for i, ax in enumerate(axes.flatten()):

    im = ax.imshow(lena)
    divider = make_axes_locatable(ax)
    if i == 2:
        ax_cb = divider.new_vertical(size="10%", pad=0.3, pack_start=True)
        fig.add_axes(ax_cb)
        fig.colorbar(im, cax=ax_cb,  orientation="horizontal")
        ax_cb2 = divider.new_vertical(size="10%", pad=0.3, pack_start=True)
        fig.add_axes(ax_cb2)
        fig.colorbar(im, cax=ax_cb2,  orientation="horizontal")
    if i == 3:
        ax_cb = divider.new_vertical(size="10%", pad=0.3, pack_start=True)
        fig.add_axes(ax_cb)
        fig.colorbar(im, cax=ax_cb,  orientation="horizontal")

plt.show()

Answer 1

Usually you don't want to use the absolute positions of the axes, because this might (as you found out) change depending on other artists drawn or the figure placed in a canvas.

If the aim is to position the colorbar below the axes, a useful way is to use mpl_toolkits.axes_grid1.make_axes_locatable as seen in this official example.

import matplotlib.pyplot as plt
import matplotlib.gridspec as gridspec
import scipy.misc
from mpl_toolkits.axes_grid1 import make_axes_locatable

fig = plt.figure(1)
fig.set_tight_layout(True)
gs = gridspec.GridSpec(2,2)
ax = plt.subplot(gs[0,0])
lena = scipy.misc.face()
im = ax.imshow(lena)

divider = make_axes_locatable(ax)

ax_cb = divider.new_vertical(size="5%", pad=0.3, pack_start=True)
fig.add_axes(ax_cb)

fig.colorbar(im, cax=ax_cb,  orientation="horizontal")

plt.show()

Note that this is absolutely compatible with tight_layout while setting an axes with add_axes is not.

You may create further errorbars by dividing the axes further using a second call to divider.new_vertical.

import matplotlib.pyplot as plt
import matplotlib.gridspec as gridspec
import scipy.misc
from mpl_toolkits.axes_grid1 import make_axes_locatable

fig, axes = plt.subplots(2,2)
fig.set_tight_layout(True)

lena = scipy.misc.face()

for i, ax in enumerate(axes.flatten()):

    im = ax.imshow(lena)
    divider = make_axes_locatable(ax)
    ax_cb = divider.new_vertical(size="10%", pad=0.3, pack_start=True)
    ax_cb2 = divider.new_vertical(size="10%", pad=0.3, pack_start=True)
    fig.add_axes(ax_cb)
    fig.colorbar(im, cax=ax_cb,  orientation="horizontal")
    if i % 2 - 1:
        fig.add_axes(ax_cb2)
        fig.colorbar(im, cax=ax_cb2,  orientation="horizontal")

plt.show()

Edit for the edited part of the question:

import matplotlib.pyplot as plt
import matplotlib.gridspec as gridspec
import scipy.misc
from mpl_toolkits.axes_grid1 import make_axes_locatable


fig, axes = plt.subplots(2,2)
fig.set_tight_layout(True)

lena = scipy.misc.face()

for i, ax in enumerate(axes.flatten()):

    im = ax.imshow(lena)
    divider = make_axes_locatable(ax)
    ax_cb = divider.new_vertical(size="8%", pad=0.3, pack_start=True)
    ax_cb2 = divider.new_vertical(size="8%", pad=0.02, pack_start=True)
    if i == 2:
        fig.add_axes(ax_cb)
        fig.colorbar(im, cax=ax_cb,  orientation="horizontal")
        fig.add_axes(ax_cb2)
        fig.colorbar(im, cax=ax_cb2,  orientation="horizontal")
        ax_cb.tick_params(bottom=False,labelbottom=False)
    if i == 3:
        fig.add_axes(ax_cb)
        fig.colorbar(im, cax=ax_cb,  orientation="horizontal")

plt.show()