R: adding multiple regression lines and loess curve to plot

Question

mod = lm(iris$Petal.Length ~ iris$Petal.Width + iris$Species)
plot(iris$Petal.Length ~ iris$Petal.Width, col = iris$Species)
abline(mod)

Here I'm stratifying Species and I want to plot 3 regression lines, one for each species. abline(mod) seems to only add one line only. Also, what if I wanted to add LOESS curve?

Answer 1

A ggplot one-liner:

library(ggplot2)

ggplot(iris, aes(Petal.Width, Petal.Length, color=Species)) + geom_point() + geom_smooth(method='lm', formula=y~x)

Leave out the arguments to geom_smooth() and you would get the LOESS line. However, the data here are so scant that this fails.

Answer 2

mod = lm(iris$Petal.Length ~ iris$Petal.Width + iris$Species)
plot(iris$Petal.Length ~ iris$Petal.Width, col = iris$Species)
abline(coefficients(mod)[1], coefficients(mod)[2])
abline(coefficients(mod)[1], coefficients(mod)[3])
abline(coefficients(mod)[1], coefficients(mod)[4])