[python-uk] Matplotlib show() doesn't display on OS X

[gvim]

I ran this simple script taken from a book:

#!/usr/bin/env python3

import matplotlib.pyplot as plt

squares = [1, 4, 9, 16, 25]
plt.plot(squares)
plt.show()


... but although the script runs without error it doesn't display 
anything, doesn't terminate and is unresponsive to either Ctrl+C or Ctrl+D.

gvim