Recently I was running a script that produced a time series plot and needed to draw some horizontal reference lines that highlighted when certain programs were implemented. Usually this is a task handled quite efficiently with the abline() function. The problem was that, due to some custom formatting (user generated legend that was outside of plotting area by using xpd=TRUE in par()), when I called abline, the plotting device would draw the line through the entire plotting surface (e.g., the line was not confined to the plotting area).

So what I had to do was use the segment() function to draw my lines. The only problem was that I needed the precise upper and lower coordinates to feed to the segment command. Even though in my case I used a user defined ylim (max and min of y), the plot adds a little bit of a fudge-factor to these upper and lower limits. Enter the usr command. My answer came in the form of this very useful R help posting. The usr option to par() returns a vector of the exact corner limit values of your plotting area (very handy!).

Here is another nice post about the ‘usr’ option to par.

Here’s how it all came together.

##  model of my problem
par(xpd=TRUE)
x <- stats::runif(12); y <- stats::rnorm(12)
i <- order(x, y); x <- x[i]; y <- y[i]
plot(x, y, main = "Notice how the line extends beyond plot margins")
abline(v=0.4)

##  my solution
par(xpd=TRUE)
x <- stats::runif(12); y <- stats::rnorm(12)
i <- order(x, y); x <- x[i]; y <- y[i]
plot(x, y, main = "Notice how the line extends beyond plot margins")
u <- par("usr")
segments(0.4, u[3], 0.4, u[4])