Question

When I do a simple qplot() I want one regression line for the plot. How do I tell stat_smooth to ignore the factors?

Here’s my example code:

library("ggplot2")
qplot(y=wt, x=mpg, size=cyl, col=factor(gear), data=mtcars) +
    stat_smooth(method=lm, formula=y~x)

That gives this image:

When I remove the factor I get the graph that I want (although I can’t remove the factor in my real dataset):

qplot(y=wt, x=mpg, size=cyl, col=gear, data=mtcars) +
    stat_smooth(method=lm, formula=y~x)

Answer 1

You can separate the points (for which you want the groupings by the cycl and gear factors) from the smoother (for which you just want the x and y aesthetics, and nothing else).

ggplot( mtcars, aes( y=wt, x=mpg ) ) + 
  geom_point( aes(size=cyl, colour=factor(gear)) ) +
  stat_smooth( method="lm" )

Or if you have a lot of geom and want to remove the default aesthetics from just one geom:

ggplot( mtcars, aes( y=wt, x=mpg, size=cyl, colour=factor(gear)) ) + 
  geom_point() +
  stat_smooth(method="lm", aes(size = NULL, colour = NULL))