Re: [R] user defined panel function

From: Sundar Dorai-Raj <sundar.dorai-raj_at_pdf.com>
Date: Tue 30 Aug 2005 - 05:44:57 EST

Martin Henry H. Stevens wrote:
> Mac OS 10.3.9 R framework v. 2.1.1
>
> I am attempting to put a fitted curve into each panel of a lattice
> graph, but am failing to do so. I have tried writing a very
> sophisticated function to do so. The function seems to work when used
> with plot(), but does not do so inside a panel function in xyplot().
> Any pointers would be appreciated.
>
> #The example data
> fact <- gl(2,7)
> x <- rep(1:7,2)
> y <- c(1,1,2,3,2,3,4,1,2,1,2,3,3,4)
> plot(jitter(y/6) ~ x)
>
> # The following user defined function puts a curve (I believe the
> correct one) into the scatterplot
> panel.predglm <- function(x, y) {
> model.trial <- glm(cbind(y,6-y) ~ poly(x,2),
> family=quasibinomial(link="logit"))
> xfit <- seq(1,7, length=21)
> yfit <- predict(model.trial, newdata=data.frame(x=xfit),
> type="response")
> lines(xfit,yfit) }
>
> panel.predglm(x, y)
>
>
> # My attempt to use it in a lattice xyplot, however, fails. It draws a
> curve which in most cases is outside the dimensions of the plot. I
> suspect that the prediction is on the scale of the link functions.
>
> library(lattice)
> xyplot(y/6 ~ x|fact, ylim=c(0,.8),
> panel=function(x, y,...) {
> panel.xyplot(jitter(x),jitter(y))
> panel.predglm(x,y) }
> )
>
> Any thoughts?

Two:

  1. The "y" argument in your panel function ranges from 0 to 1 and not 0 to 6 as your plot example assumes.
  2. You need to use llines in your panel function and not lines.

Here's a working example:

library(lattice)

fact <- gl(2,7)
x <- rep(1:7,2)
y <- c(1,1,2,3,2,3,4,1,2,1,2,3,3,4)

# The following user defined function puts a curve (I believe the correct one) into the scatterplot
panel.predglm <- function(x, y) {

	model.trial <- glm(cbind(y,6-y) ~ poly(x,2),
                          family=quasibinomial(link="logit"))
	xfit <- seq(1, 7, length=21)
	yfit <- predict(model.trial, newdata=data.frame(x=xfit), type="response")
	llines(xfit,yfit)

}

xyplot(y/6 ~ x|fact, ylim=c(0,.8),

        panel = function(x, y, ...) {
          panel.xyplot(jitter(x), jitter(y))
          panel.predglm(x, y * 6)
        })

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