ggplot2 - How to add legend to geom_smooth in ggplot in R

Question

Have a problem of adding legend to different smooth in ggplot.

    library(splines)
    library(ggplot2)
    temp <- data.frame(x = rnorm(200, 20, 15), y = rnorm(200, 30, 8))

    ggplot(data = temp, aes(x, y)) + geom_point() + 
      geom_smooth(method = 'lm', formula = y ~ bs(x, df=5, intercept = T), col='blue') + 
      geom_smooth(method = 'lm', formula = y ~ ns(x, df=2, intercept = T), col='red')

I have two splines: red and blue. How I can add a legend for them?

Answer 1

Put the colour in aes() and add scale_colour_manual():

ggplot(data = temp, aes(x, y)) + geom_point() + 
  geom_smooth(method = 'lm', formula = y ~ bs(x, df=5, intercept = T), aes(colour="A")) + 
  geom_smooth(method = 'lm', formula = y ~ ns(x, df=2, intercept = T), aes(colour="B")) +
  scale_colour_manual(name="legend", values=c("blue", "red"))