[R] ploting 2 variables in a lattice xyplot

[Ernesto Jardim]

Hi,

I want to plot 2 variables (confidence intervals) in a single xyplot. I
have a dataframe with variables Yup, Ylo, X and Z and I want to have a
xyplot with both variables ploted. I'm trying with 

xyplot(Yup~X|Z, panel=function(x){...})

but this way I'm not able to pass variable Ylo into the function...

How can I do this ?

Thanks

EJ