When a colorbar is used in a Matplotlib plot, it is usually drawn with a height that is larger than the plot itself. Many solutions to this problem are suggested here. The one that worked for me was this:
plt.colorbar(im, fraction=0.046, pad=0.04)
This sets the colorbar height equal to the height of the plot for me.
Tried with: Matplotlib 1.3.1-1, Python 2.7.6 and Ubuntu 14.04