PGF backend subplot ylabel alignment bug

pgf_subplot_ylabel_align_test.py

"""
    +-----+                    +-----+
  a |     |                a   |     |
  x | ax1 |                x   | ax1 |
  1 |     |                1   |     |
    +-----+ should become:     +-----+
a   |     |                a   |     |
x   | ax2 |                x   | ax2 |
2   |     |                2   |     |
    +-----+                    +-----+
"""

from sys import argv

# To avoid issue #2037, correct backend has to be loaded here
import matplotlib
if len(argv) > 1:
    matplotlib.use('pgf')
else:
    matplotlib.use('Agg')
import matplotlib.pyplot as plt
import matplotlib.transforms as tfs
import numpy as np


def align_ylabels(event):
    fig = event.canvas.figure
    axes = fig.axes
    # Get desired (=smallest) x value
    xpos = min(ax.yaxis.get_label().get_position()[0] for ax in axes)
    for ax in axes:
        # get_position() returns xpos in display coordinates and ypos in
        # axes coordinates, this transform accounts for that
        trans = tfs.blended_transform_factory(tfs.IdentityTransform(), ax.transAxes)
        # Update label coordinates
        ax.yaxis.set_label_coords(xpos, 0.5, transform=trans)
    # Following tidbit stolen from http://stackoverflow.com/a/4056853/1145828
    # Remove callbacks before redrawing figure with updated coordinates,
    # to avoid recursion
    func_handles = fig.canvas.callbacks.callbacks[event.name]
    fig.canvas.callbacks.callbacks[event.name] = {}
    # Redraw figure
    fig.canvas.draw()
    # Reset callbacks
    fig.canvas.callbacks.callbacks[event.name] = func_handles


f = plt.figure()
f.canvas.mpl_connect('draw_event', align_ylabels)

ax1 = f.add_subplot(211, ylabel='ax1')
ax2 = f.add_subplot(212, ylabel='ax2')

# y1 and y2 are of different width so the top and bottom labels are
# intentionally misaligned
x = np.linspace(0, 10)
y1 = np.linspace(0, 10)
y2 = np.linspace(1000, 10000)

ax1.plot(x, y1)
ax2.plot(x, y2)

if len(argv) > 1:
    # Use PGF backend to save PDF file
    # ylabel alignment doesn't work
    f.savefig('img.pdf')
else:
    # Use Agg backend to save PNG file
    # ylabel alignment works
    f.savefig('img.png')