How to force axis to have the same range

I am trying to produce (standard and trellis) scatterplots which use the 
same range of the x and y axes. In addition, I would like the plots to 
be physically square. Is there one or more specific argument(s) to plot 
and xyplot what would do that? I have looked at various combinations of 
asp and pin value, but could not get what I wanted..