[Matplotlib-users] contour lines not hidden by patches

Can't figure this out: I create a figure, add some axes, define data to be plotted as a contourf + contour on top and then add some patches to hide some regions of my plot.
The patches hide the contourf correctly, as expected, but not the contour lines...
Could someone telle me whether I'm doing something wrong ?
Below are the code lines.
I'm using python 2.4, matplotlib 0.91.2

Thanks in advance,

Aure


#----------
    pylab.clf()

    fig = pylab.figure(figsize=(7.,3.8),dpi=100,facecolor='white')

    axes1 = fig.add_axes((.....),label='axes1')
    axes2 = fig.add_axes((.....),label='axes2')            
    axes3 = fig.add_axes((.....),label='axes3')            

    #define contour data
    contourstep = 0.05
    contourx  = np.arange(0.01,2.5+contourstep,contourstep)
    contoury = np.arange(0.01,4.+contourstep,contourstep)
    contourxy1 = np.ones((len(contoury),len(contourx)))
    contourxy2 = np.ones((len(contoury),len(contourx)))
    x = np.arange(0.,6.,0.01)
    for j in range(contourxy1.shape[0]):
        for k in range(contourxy1.shape[1]):
            newval1=...
            newval2=...
            contourxy1[j,k] = newval1
            contourxy2[j,k] = newval2
                
    #add contour
    levels = np.arange(0.0,0.85,0.05)
    cf1 = axes1.contourf(-contourx,contoury,contourxy2,levels,cmap=bone_r,extend = 'max')   #cmap=mpl.cm.gray_r)
    cf2 = axes2.contourf(contourx,contoury,contourxy1,levels,cmap=bone_r,extend = 'max')   #cmap=mpl.cm.gray_r)
    levels2 = np.arange(0.,.45,0.05)
    axes1.contour(-contourx,contoury,contourxy2,levels2,colors='gray')
    axes2.contour(contourx,contoury,contourxy1,levels2,colors='gray')

    #add patches
    axes1.add_patch(mpl.patches.Polygon([(-1.,1),(-2.,1.),(-2.,2.)],
                                        edgecolor='k',
                                        facecolor='w',
                                        ))
    axes1.add_patch(mpl.patches.Polygon([(-1.,1.),(0.,0.),(0.,1.)],
                                        edgecolor='k',
                                        facecolor='w',
                                        ))
    axes2.add_patch(mpl.patches.Polygon([(0.,1.),(2.,1.),(2.,2.),(0.,2.)],
                                        edgecolor='k',
                                        facecolor='w',
                                        ))

    pylab.savefig(outputfilename)
#----------