interpolation on irregularly spaced grids

Trying to set the interpolation method for a NonUniformImage object I receive the following error:

>>> im.set_interpolation('bilinear')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/site-packages/matplotlib/", line 396, in set_interpolation
    raise NotImplementedError('Only nearest neighbor supported')

If I am not mistaken according to the class reference on the web-page this should be possible though. Is this functionality depending on the version I am running (0.91.2) or am I doing something wrong

Alternatively, what methods are there to obtain interpolated color plots of data on irregurarly spaced rectengular grids?