extend geom_smooth in a single direction

Question

You can extend a regression line in ggplot2 pretty easily:

c <- ggplot(mtcars, aes(y=wt, x=mpg)) + xlim(0,50)
c + stat_smooth(method=lm, fullrange=TRUE) + geom_point()

My question is, is it possible to extend in one direction only?

Before you ask, I have a good reason to do this, I promise!

Answer 1

In the internal workings of stat_smooth, predictdf is called to create the smoothed line. The difficulty here is : This is an S3 method not exported. It also don't take ... parameters so it is really difficult to extend it.

Here the idea is to create a new dummy classes lm_right and lm_left where we call the default lm method.

## decorate lm object with a new class lm_right
lm_right <- function(formula,data,...){
  mod <- lm(formula,data)
  class(mod) <- c('lm_right',class(mod))
  mod
}

## decorate lm object with a new class lm_left
lm_left <- function(formula,data,...){
  mod <- lm(formula,data)
  class(mod) <- c('lm_left',class(mod))
  mod
}

Then for each method we create a predict_df specialization where we truncate the x values in the opposite side.

predictdf.lm_right <- 
  function(model, xseq, se, level){
    ## here the main code: truncate to x values at the right
    init_range = range(model$model$x)
    xseq <- xseq[xseq >=init_range[1]]
    ggplot2:::predictdf.default(model, xseq[-length(xseq)], se, level)
  }

Same thing for the left extension :

predictdf.lm_left <- 
  function(model, xseq, se, level){
    init_range = range(model$model$x)
    ## here the main code: truncate to x values at the left
    xseq <- xseq[xseq <=init_range[2]]
    ggplot2:::predictdf.default(model, xseq[-length(xseq)], se, level)
  }

Finally a using example:

library(ggplot2)
library(gridExtra)
## you should set the fullrange option to a true 
p1 <- ggplot(mtcars, aes(y=wt, x=mpg)) + xlim(0,50) +  geom_point() +
     stat_smooth(method="lm_left", fullrange=TRUE,col='green') 
p2 <- ggplot(mtcars, aes(y=wt, x=mpg)) + xlim(0,50) +  geom_point() +
  stat_smooth(method="lm_right", fullrange=TRUE,col='red') 

grid.arrange(p1,p2)