Add a new variable to a new geom

[talegari]

How do we add a new variable to a new geom?
Here is the example from the ggplot2's vignette. I intend to add weight variable as an aesthetic with 1 as default.

Unsuccessful attempt:

StatLm <- ggproto("StatLm", Stat, 
  # required_aes = c("x", "y"),
  required_aes = c("x", "y", "w"),
  
  compute_group = function(data, scales) {
    rng <- range(data$x, na.rm = TRUE)
    grid <- data.frame(x = rng)
    
   #  mod <- lm(y ~ x, data = data)
    if (w == 1) {
        mod <- lm(y ~ x, data = data)
    } else {
        mod <- lm(y ~ x, weight = data$w, data = data)
    }
    grid$y <- predict(mod, newdata = grid)
    
    grid
  }
)

stat_lm <- function(mapping = NULL, data = NULL, geom = "line",
                    position = "identity", na.rm = FALSE, show.legend = NA, 
                    inherit.aes = TRUE, ...) {
  layer(
    stat = StatLm, data = data, mapping = mapping, geom = geom, 
    position = position, show.legend = show.legend, inherit.aes = inherit.aes,
    params = list(na.rm = na.rm, ...)
  )
}

ggplot(mpg, aes(displ, hwy)) + 
  geom_point() + 
  stat_lm()

This leads to error:

Error in layer(stat = StatLm, data = data, mapping = mapping, geom = geom,  : 
  object 'w' not found