x11() x <- 1:10 y <- x^2 plot(x, y) abline(h = mean (y), col='blue') # h stands for horizontal abline(h = median(y), col='red' ) z <- locator(1) # wait for mouse click or enter pressed x <- seq(-5, 5, 0.5) y <- 0.4*x*x + 4*x + 3 + runif(length(x), -2, 2) plot(x,y) fit <- lm(y~x) abline(fit, col=2) z <- locator(1) # wait for mouse click or enter pressed