[R] Regression lines

[Tom Backer Johnsen]

My simpleminded understanding of simple regression is that when 
plotting regression lines for x on y and y on x in the same plot, the 
lines should cross each other at the respective means.  But, given the 
R function below, abline (lm(y~x)) works fine, but abline (lm(x~y)) 
does not.  Why?

function () {
attach (attitude)
x <- rating
y <- learning
detach (attitude)
plot (x, y)
abline(v=mean(x))
abline(h=mean(y))
abline (lm(y~x))
abline (lm(x~y))
}