the line determined by two points

Line <- function(x,y)
{
# x, y are vectors of length 2
# computes equation of line passing through two points (x[i],y[i])
#
B <- (y[2]-y[1])/(x[2]-x[1])
A <- y[1] - B*x[1]
c(A,B)
}

QQline <- function(x,y,...){
Y <- quantile(y[!is.na(y)], c(0.25, 0.75))
X <- quantile(x[!is.na(x)], c(0.25, 0.75))
abline(Line(X,Y),...)
}