[R] differing scales for lattice plot panels

[Joe Bloggs]

Hi,
   I want to compare densities across several groups with: 
densityplot(~values | groups, pch=".")
However one of the groups has an almost point-like distribution. 
This forces the y-axis scale to be so large that the other distribution curves are all flat in comparison and get lost amongst the data dots.
How can I produce a lattice plot with different y-axes (preferably automatically scaled) for each panel?