[R] Extrapolated regression lines

[Duncan Murdoch]

On 10/12/2006 9:41 AM, Johan A. Stenberg wrote:
> Dear list members,
> 
> When I create a simple scatterplot with a regression line (se below) the 
> line is automatically extrapolated outside the range of data points. Why 
> is this and how can I prevent R from extrapolating the regression line?
> 
> Thank you in advance,
> Johan
> 
> model<-lm(Herb~Para)
> plot(Para,Herb)
> abline(model)

abline draws a line, not a line segment.  If you want a segment that 
stays within the data, you'll need something like this:

model <- lm(Herb ~ Para)
plot(Para, Herb)
lines(Para, predict(model))

Note that if Para is not sorted, this may look a little strange, and it 
will look like a complete mess if you try to fit a polynomial.  You can 
sort it (and Herb correspondingly) by

o <- order(Para)
Para <- Para[o]
Herb <- Herb[o]

Duncan Murdoch