You want to add lines from a fitted regression model to a scatter plot.

To add a linear regression line to a scatter plot, add stat_smooth() and tell it to use method=lm. This instructs it to fit the data with the lm() (linear model) function. First we’ll save the base plot object in sp, then we’ll add different components to it:

library(gcookbook) # For the data set

# The base plot
sp <- ggplot(heightweight, aes(x=ageYear, y=heightIn))

sp + geom_point() + stat_smooth(method=lm)

By default, stat_smooth() also adds a 95% confidence region for the regression fit. The confidence interval can be changed by setting level, or it can be disabled with se=FALSE (Figure 5-18):