I've written a simple function to draw a regression line in a plot and
annotate the line showing the slope
with a label. It works, as I'm using it, when the horizontal variable
is 'x', but gives incorrect results otherwise.
What's wrong?
# simple function to show the slope of a line
show.beta <- function(model, x="x", x1, x2, label, col="black", ...) {
abline(model, col=col, lwd=2)
# x <- deparse(substitute(x)) # doesn't help
xs <- data.frame(x=c(x1, x2, x2))
ys <- predict(model, xs)
lines(cbind(xs,ys[c(1,1,2)]), col=col)
text(x2, mean(ys[1:2]), label, col=col, ...)
}
x=rnorm(10)
DF <- data.frame(x, y=.25*x+rnorm(10))
# OK
with(DF, {
plot(y ~ x)
mod <- lm(y ~ x)
show.beta(mod, "x", -0.5, 0, "b", pos=4)
})
# not OK
xx=rnorm(10)
DF2 <- data.frame(xx, y=.25*x+rnorm(10))
with(DF2, {
plot(y ~ xx)
mod <- lm(y ~ xx)
show.beta(mod, "xx", -0.5, 0, "b", pos=4)
})
From the latter, I get:
Warning message:
'newdata' had 3 rows but variable(s) found have 10 rows
>
--
Michael Friendly Email: friendly AT yorku DOT ca
Professor, Psychology Dept.
York University Voice: 416 736-5115 x66249 Fax: 416 736-5814
4700 Keele Street Web: http://www.datavis.ca
Toronto, ONT M3J 1P3 CANADA
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