Meaning of band width in ggplot geom_smooth lm

Question

With the following code:

library(ggplot2)
ggplot(mtcars, aes(x=wt, y=mpg)) +
    geom_point(aes(colour=factor(cyl))) +
    geom_smooth(method="lm")

I can get this plot:

My question is how does the grey zone defined? What's the meaning of it. And how can I play around with various parameter that control the width of that band?

Answer 1

It's the confidence interval. You can use se=FALSE if you do not want to display it. You can also use level = 0.99 if you want to have a 99% CI instead of a 95% CI. See ?stat_smooth for all the details.

Answer 2

By default, it is the 95% confidence level interval for predictions from a linear model ("lm"). The documentation from ?geom_smooth states that:

The default stat for this geom is stat_smooth see that documentation for more options to control the underlying statistical transformation.

Digging one level deeper, doc from ?stat_smooth tells us about the methods used to calculate the smoother's area.

For quick results, one can play with one of the arguments for stat_smooth which is level : level of confidence interval to use (0.95 by default)

By passing that parameter to geom_smooth, it is passed in turn to stat_smooth, so that if you wish to have a narrower region, you could use for instance .90 as a confidence level:

ggplot(mtcars, aes(x=wt, y=mpg)) +
    geom_point(aes(colour=factor(cyl))) +
    geom_smooth(method="lm", level=0.90)