[R] How to set the limit of abline (regression line of lm)

[Gavin Simpson]

On Mon, 2010-09-20 at 17:31 +0800, elaine kuo wrote:
> Dear List,
> 
> 
> 
> I ran a regression model using lm and produced a regression line using
> abline.
> 
> 
> The line ranges from -20 to 20 in x axis,
> 
> and the section I only want is from -20 to 0.
> 
> 
> 
> Please kindly advise any function in abline () to set the range of x axes.

You can't; these are already set by the plot, and abline only *adds* to
the plot. You need to think about this a different way. What you want
are the model predictions for points spread over the range (-20, 0]. So
set up a new data set of 'x' values and use predict() to generate the
appropriate 'y' values.

Here's an example:

set.seed(123)
dat <- data.frame(x = sample(seq(-20, 20, length = 100)))
dat <- within(dat, y <- 8 + (1.8 * x) + (10 * rnorm(100)))
mod <- lm(y ~ x, data = dat)

## now plot the data
plot(y ~ x, data = dat)

## now predict 50 values spread evenly on range (-20, 0]
newdat <- data.frame(x = seq(-20, 0, length = 50))
newdat <- within(newdat, ypred <- predict(mod, newdat))

## add these predictions as a line to the plot
lines(ypred ~ x, data = newdat, col = "red", lwd = 2)

HTH

G

> Thank you
> 
> 
> 
> Elaine
> 
> 	[[alternative HTML version deleted]]
> 
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