ax2.patch.set_facecolor('gray')
ax2.plot([0, 1], [0, 3], color='red')
ax1 = f.add_axes([0.1, 0.1, 0.8, 0.8], zorder=2)
ax1.plot([0, 1], [0, 1], color='blue')
ax1.patch.set_alpha(0.0)
Matplotlib: Overplot a line above another axes panel