[R] error in packet 1

[Giovanni Tarquinio]

Hello, 
I'm Giovanni from ROMA..

I can't find a solution for the error:

"error using packet 1
the y field is not specified and it has not a default value"
(this is my traslation from italian language)

The code is:

pc<-    qqmath(~valori,
        distribution=function(p) qweibull(p,beta,alpha),
        prepanel = prepanel.qqmathline,
        panel = function(x, y) {
             panel.grid()
             panel.qqmathline(y, distribution = function(p)
qweibull(p,beta,alpha))
             panel.qqmath(x, y)
           },
        layout = c( 1,1), aspect = 0.8,
        xlab = "Unit Weibull Quantile", ylab = "D"
    )


Thanks for your regard,
Giovanni