Superpose two QQ-plots (gamma distribution) with, lattice function qqmath()

David, Duncan,

Hi

Following on David's rate argument

try (with modifications of pch and grid)

rate <- 1/4
shape = 8
rate = c(rep(1/4,100),rep(1/3,100))

I don't think the problem is related to the rate argument, which can 
well be vectorized, as is the case for a number of arguments in 
distrib functions in R (note that you are redefining it as a vector above).

x = rgamma(200,shape,rate)
groups = gl(2,100,200,labels=LETTERS[1:2])
dat = data.frame(x=x, gp=groups)

qqmath(~ x,  data = dat,
         groups = gp,
         pch = 20,
         type = c("p","g"),
         distribution = function(x) qgamma(x,shape,rate),
         panel = function(x,groups,...) {
                   panel.qqmath(x,groups, ...)
         })

This works! Thank you very much (I had spent some time on that).

I now try to add a QQ-line but face a new problem. Only one line 
appears, away from the points:
qqmath(~ x,  data = dat,
        groups = gp,
        pch = 20,
        type = c("p","g"),
        distribution = function(x) qgamma(x,shape,rate),
        panel = function(x,groups,...) {
                  panel.qqmath(x,groups, ...)
                  panel.qqmathline(x,groups,...)
        })

Any hint?

Thanks a lot for your help,

Yvonnick