[R] superpose two variables in lattice/xyplot

[Arien Lam]

Dear R users,

I try to use xyplot() to display two different response variables from 
the same dataframe per panel, but don't succeed:

xyplot(ptot.seaslog ~ vmcwit | seas, data=reeks, as.table=TRUE,
        panel =  function(x,y){
          panel.xyplot(x, y, ylim=c(0,1))
          panel.superpose(x=reeks$vmcwit, y=reeks$ptotbin,
            panel.groups = "panel.xyplot", groups=reeks$seas,
            subscripts = TRUE, col="red" )
        }
)

The second respons variable "reeks$ptotbin" is plotted, but is not 
subset for each of 4 panels. How can it be done?

Any suggestions warmly appreciated.

Kind regards, Arien