How to annotate a reference line at the same angle as the reference line itself?

Question

I need to find a way to annotate a reference line at the same angle as the reference line itself.

The following statement will produce the reference line and the la

Answer 1

An addendum to @Andrie's answer: instead of hard-coding the aspect ratio in the plot the first time to get the relative coordinate scales, you can recover the current working aspect ratio with the following function:

getCurrentAspect <- function() {
   uy <- diff(grconvertY(1:2,"user","inches"))
   ux <- diff(grconvertX(1:2,"user","inches"))
   uy/ux
}

So you can create your plot: set asp <- getCurrentAspect(); and proceed with the rest of @Andrie's solution.

For all I know this function exists somewhere in the R ecosystem, but I haven't seen it ...