ggplot2 in R: fill underneath a geom_smooth line

Question

I am trying to fill in a portion of a plot underneath a geom_smooth() line.

Example:

In the example the data fits on that curve. My data is not as smooth. I want to use geom_point() and a mix of geom_smooth() and geom_area() to fill in the area under the smoothed line while leaving the points above.

A picture of my data with a geom_smooth():

In other words, I want everything underneath that line to be filled in, like in Image 1.

Answer 1

Use predict with the type of smoothing being used. geom_smooth uses loess for n < 1000 and gam for n > 1000.

library(ggplot2)

ggplot(mpg, aes(displ, hwy)) +
    geom_point() +
    geom_smooth() +
    geom_ribbon(aes(ymin = 0,ymax = predict(loess(hwy ~ displ))),
                alpha = 0.3,fill = 'green')

Which gives: