plot linear regressions lines without interaction

Question

This code plots regression lines with interactions in ggplot2:

library(ggplot2)
ggplot(mtcars, aes(hp, mpg, group = cyl)) + geom_point() + stat_smooth(method = "lm")

Can lines without interactions be plotted with stat_smooth?

Answer 1

Workaround would be to make model outside the ggplot(). Then make predicition for this model and add result to the original data frame. This will add columns fit, lwr and upr.

mod<-lm(mpg~factor(cyl)+hp,data=mtcars)
mtcars<-cbind(mtcars,predict(mod,interval="confidence"))

Now you can use geom_line() with fit values as y to add three regression lines and geom_ribbon() with lwr and upr to add confidence interval.

ggplot(mtcars, aes(hp, mpg, group = cyl)) + geom_point() +
      geom_line(aes(y=fit))+geom_ribbon(aes(ymin=lwr,ymax=upr),alpha=0.4)