library(ggplot2)
p <- qplot(x = wt, y = mpg, data = mtcars)
p + geom_abline()

p + geom_abline(intercept = 20)

coefficients <- coef(lm(formula = mpg ~ wt, data = mtcars))
p + geom_abline(intercept = coefficients[["(Intercept)"]], 
                slope = coefficients[["wt"]])

coefficients <- coef(lm(formula = mpg ~ wt, data = mtcars))
p + geom_abline(intercept = coefficients[["(Intercept)"]], 
                slope = coefficients[["wt"]], color = "red", 
                size = 2)

p + stat_smooth(method = "lm", se = TRUE)

p + stat_smooth(method = "lm", se = FALSE)

p <- ggplot(data = mtcars, mapping = aes(x = wt, y = mpg)) + 
    geom_point()
df <- data.frame(a = rnorm(n = 10, mean = 25), 
                 b = rnorm(n = 10, mean = 0))
p + geom_abline(mapping = aes(intercept = a, slope = b), data = df)

p + geom_smooth(mapping = aes(group = cyl), method = "lm")

p + geom_smooth(mapping = aes(group = cyl), method = "lm", 
                fullrange = TRUE)

p + geom_abline(intercept = coefficients[["(Intercept)"]], 
                slope = coefficients[["wt"]]) + coord_flip()

p + geom_abline(intercept = coefficients[["(Intercept)"]], 
                slope = coefficients[["wt"]]) + coord_polar()