# Add vertical line
plot(cars)
abline(v=15, col="blue")
# add 2 vertical lines: change line colors, sizes and types
plot(cars)
abline(v=c(15,20), col=c("blue", "red"), lty=c(1,2), lwd=c(1, 3))
# add an horisontal line
abline(h=40, col="blue")