Lattice - parallel: xlim and adding lines

Hi,

I'm trying to set the minimum and maximum in a parallel 
plot, but trying xlim=c(-1,1) gives strange results. Am I 
missing something? The call I give is:

parallel(~X[,c(6,9,12,15,18)]|X$ff,X,panel=panel.parallel.new,groups=X$protein,layout=c(3,1),xlim=c(-1,1))

Besides, I would like to add a reference line to the plot, 
but can't find how to do that.

I hope someone can give me a hint or two.

Thanks in advance,

Tsjerk