4.7 Adding best fit lines

It is often useful to summarize the relationship displayed in the scatterplot, using a best fit line. Many types of lines are supported, including linear, polynomial, and nonparametric (loess). By default, 95% confidence limits for these lines are displayed.

# scatterplot with linear fit line
ggplot(Salaries,
       aes(x = yrs.since.phd, 
           y = salary)) +
  geom_point(color= "steelblue") +
  geom_smooth(method = "lm")

## `geom_smooth()` using formula = 'y ~ x'

# scatterplot with quadratic line of best fit
ggplot(Salaries, 
       aes(x = yrs.since.phd, 
           y = salary)) +
  geom_point(color= "steelblue") +
  geom_smooth(method = "lm", 
              formula = y ~ poly(x, 2), 
              color = "indianred3")