contoured "slice" in 3D scatterplot


I'm running matplotlib v 0.99.0.

I can create a 3D scatterplot, like this:

import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D

fig = plt.figure()
ax = Axes3D(fig)
ax.scatter( ... )

so far so good.

Now, say I want to add a 2D contourplot (a "slice" of the 3D scatter contoured).
Is there some way how I can specify along which coordinates the contourplot is plotted, or is it always plotted on the (x,y) plane?

How I have tried this is by
from matplotlib.mlab import griddata

fi = griddata( ... )
ax.contourf(x,z,fi, ... )
# even though I specify z, it plots the contourf along the y axis.

Thank you all for any pointers,