Suggestion for qqplot() improvement

[Prof Brian D Ripley]

(Martin Maechler quoting Werner Stahel, I think)

> Maybe even
>       qqplot(qt(ppoints(n), df=5), y, 
> 	     xlab='quantiles of t(* ,df=5)', ylab='data')
> 
>     WSt> and this could be shortened to
> 
>     WSt>   qqplot(y, distribution='t', df=5)
> 
>     WSt> with an easy extension of the function.

Would qqplot(qt, y, df=5) or qqplot("t", y, df=5) with the following do
what you want?  As plot warns about surplus arguments, I have removed them
from the ... in its call.

qqplot <- function (x, y, plot.it = TRUE, xlab = deparse(substitute(x)), 
    ylab = deparse(substitute(y)), ...) 
{
    xlab
    sy <- sort(y)
    leny <- length(sy)
    dots <- list(...)
    if(is.character(x)) x <- get(paste("q", x, sep=""))
    if(is.function(x)) {
        fn <- x
        formals(fn) <- c(formals(fn), "..." = list(NULL))
        sx <- fn(ppoints(leny), ...)
        dots <- dots[!match(names(dots), names(formals(x)), FALSE)]
    } else {    
        sx <- sort(x)
        lenx <- length(sx)
        if (leny < lenx) sx <- approx(1:lenx, sx, n = leny)$y
        if (leny > lenx) sy <- approx(1:leny, sy, n = lenx)$y
    }
    if (plot.it) 
        do.call("plot", c(list(sx, sy, xlab = xlab, ylab = ylab), dots))
    invisible(list(x = sx, y = sy))
}


-- 
Brian D. Ripley,                  ripley@stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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