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

Working example for ggplot2:

slope<-1.3
asp<-1
p <- ggplot()
p<-p+scale_x_continuous(limits = c(1, 15), expand = c(0, 0)) 
p<-p+scale_y_continuous(limits = c(-8, 20), expand = c(0, 0)) 
p<-p+geom_abline(intercept = 0, slope = slope)
p<-p+coord_fixed(asp)
p<-p+annotate("text", label = "reference line label", x = 3.5, y = 5,  colour = "red",angle=atan(slope*asp)*180/pi)
p

text with angle:

Answer 2

A similar solution with ggplot2

data <- data.frame(x = 1:10, y = 1:10)
intercept <- 10
slope <- -1
ggplot(data, aes(x,y)) + geom_point(shape=1) +  
  geom_abline(intercept = intercept, slope = slope) + 
  geom_text(x=4, y=5, label="my label", angle=atan(slope)*180/pi)

intercept <- 10
slope <- -2
ggplot(data, aes(x,y)) + geom_point(shape=1) +  
  geom_abline(intercept = intercept, slope = slope) + 
  geom_text(x=4, y=5, label="my label", angle=atan(slope)*180/pi)

Answer 3

One way is by setting the plot aspect ratio, using the asp argument, and then to calculate the angles using the specified asp:

asp <- 2

plot(1:10,1:10, asp=asp) 
abline(a=8, b=-1)
text(x=4, y=5, "reference line label", srt=180/pi*atan(-1*asp))

abline(a=4, b=-2)
text(x=1, y=3, "reference line label", srt=180/pi*atan(-2*asp))

Setting a different asp:

asp <- 0.8

plot(1:10,1:10, asp=asp) 
abline(a=8, b=-1)
text(x=4, y=5, "reference line label", srt=180/pi*atan(-1*asp))

abline(a=4, b=-2)
text(x=1, y=3, "reference line label", srt=180/pi*atan(-2*asp))

Answer 4

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 ...