Create a figure and axis, and add a vspan to that axis. If you twinx it, and apply a plot to the twin, the x_limits can adjust, potentially making the vspan not visible.

Minimal code sample:

fig, ax = plt.subplots(figsize=(12, 10))
ax.axvspan(1,2)
fig.canvas.draw()

bx = ax.twinx()
bx.plot([0,.5], [1,2])
fig.canvas.draw()

Note how the x-axis labels shifted away from the range (1,2).
I expected:

• Matplotlib version, Python version and Platform (Windows, OSX, Linux ...)
  Matplotlib 1.5.1 on OS X 10.11 with Python 2.7.10
• System python, pip install -U matplotlib (with SIP disabled)