[Matplotlib-users] Working with background image in plot

Hello!

I try to use a background image in my plot:

import matplotlib.pyplot as plt
im = plt.imread("WTunnel.png")
plt.xlim([-3,4])
plt.ylim([-0.01,0.05])
plt.plot(some stuff)
plt.imshow(im, aspect="auto", extent=(-5, 4, -0.01, 0.05))

This works more or less.

As far as I understand the extent argument works as scaling factor. Is there a 
way to move the background picture? I need to exactly set the x position since 
the background image is associated with the x data from the plots.

The origin argument seems to be promising, but it accepts only 
[None|‘upper’|‘lower’].

What is the best way to handle such a picture?

Background: The data is pressure measurements over x. The background image 
shows the geometric features of the measured object. Exact x_0 and dx is 
crucial, y values of the picture just need to look good ;-)

Thanks!

Florian