[R] roc and lattice

[rhk]

Hello, I am afraid I do not fully understand all intricacies of 
programming in lattice plots. In the code below I try to plot an ROC 
curve, following R-news 4(1). When I condition on the variable 'group' I 
get the error message below, when I plot the curve for all data (i.e., y 
~ pred.prob), I get the plot I want. Can someone point out why 
conditioning gives that message? Thanks, Ruud

 > plot.a <- xyplot(y ~ pred.prob|group, data=x.df,
+  xlim=c(0,1),xlab="1-specificiteit",
+  ylab="sensitiviteit",
+  panel=function(x,y,subscripts,...){
+   DD <- table(-x,y)
+   sens <- cumsum(DD[,2])/sum(DD[,2])
+   mspec <- cumsum(DD[,1])/sum(DD[,1])
+   panel.xyplot(mspec,sens,type="l",...)
+   panel.abline(0,1)
+  })
 > print(plot.a)
Error in panel(x = c(0.000265710002003069, 0.000345712857778025, 
0.000265710002003069,  :
        subscript out of bounds