How to add a trend line to a scatter plot in R

March 14, 2026 · 1 min read · Updated March 14, 2026 · beginner

r ggplot2 visualization regression statistics

Adding trend lines reveals relationships between variables in scatter plots.

With ggplot2

The geom_smooth() function adds trend lines with confidence intervals:

library(ggplot2)

ggplot(mtcars, aes(x = wt, y = mpg)) +
  geom_point() +
  geom_smooth(method = "lm", se = TRUE)

Use method = "loess" for smoothed lines, or se = FALSE to hide confidence intervals:

ggplot(mtcars, aes(x = wt, y = mpg)) +
  geom_point() +
  geom_smooth(method = "loess", se = FALSE)

For polynomial trends, specify the formula:

ggplot(mtcars, aes(x = wt, y = mpg)) +
  geom_point() +
  geom_smooth(method = "lm", formula = y ~ poly(x, 2))

Use abline() with lm() for linear regression:

plot(mtcars$wt, mtcars$mpg)
abline(lm(mtcars$mpg ~ mtcars$wt), col = "red")