Usually if you plot histogram + axvline, the x axis limits are extended so the vertical line is visible.

E.g.

But sometimes the x axis is not extended to be able to show a vertical line.
Here's the code to demonstrate the issue:

from matplotlib import pylab as plt
plt.hist([9138, 9138, 9133, 9137, 9134, 9128, 9133, 9134, 9134, 9142])
plt.axvline(x=9144, color='red')
plt.show()

(the red vertical line should be visible on the right, x=9144)

• Matplolib: 1.5.1-1ubuntu1 (from repository), Python 2.7.12, Ubuntu 16.04.1
• The same problem can be reproduced in python 3.5.2.