# Re: [R] lattice with predicted values

From: Deepayan Sarkar <deepayan.sarkar_at_gmail.com>
Date: Sat 15 Oct 2005 - 07:24:22 EST

On 10/14/05, Martin Henry H. Stevens <HStevens@muohio.edu> wrote:
> Dear lattice wizards,
>
> I am trying to figure out how to plot predicted values in xyplot,
> where the intercept, but not the slope, varies among conditioning
> factor levels. I am sure it involves the groups, but I have been
> unsuccessful in my search in Pinhiero and Bate, in the help files, or
> in the archive, or in my attempts on my own.
>
> My example follows:
>
> FACT is a factor with levels a,b,c
> COV is the covariate
>
> mod ~ lm(Y ~ COV + FACT)
>
>
> #The following draws the right predictions if the relation is the
> same for all factor levels, but I can't figure out how to have the
> same slopes but different intercepts.
>
> # Function to draw predictions in xyplot
>
> panel.predfinal <- function(mod, x, y) {
> xfit <- seq(min(x), max(x), length=21)
> yfit <- predict(mod, newdata=data.frame(COV=xfit))
> llines(xfit,yfit,lty=1) }
>
> xyplot(Y ~ COV | FACT,
> panel=function(x,y,groups,subscripts){
> panel.xyplot(x,y)
> panel.predfinal(mod,x,y) }

A not very satisfactory (but probably good enough for linear fits) is

pred <- predict(mod)

xyplot(Y ~ COV | FACT, pred = pred,

```       panel = function(x, y, pred, subscripts, ...) {
panel.xyplot(x,y,...)
llines(x, pred[subscripts], ...)
})

```

Deepayan

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