Insert Layer underneath existing layers in ggplot2 object

Expanding on Ricardo' answer, I use the following snippet:

library(ggplot2)

`-.gg` <- function(plot, layer) {
    if (missing(layer)) {
        stop("Cannot use `-.gg()` with a single argument. Did you accidentally put - on a new line?")
    }
    if (!is.ggplot(plot)) {
        stop('Need a plot on the left side')
    }
    plot$layers = c(layer, plot$layers)
    plot
}

As it allows you to:

P <- ggplot(data=dat, aes(x=id, y=val)) + geom_point()

P - geom_boxplot()

And the boxplot will be placed below the points.

Thanks @baptiste for pointing me in the right direction. To insert a layer underneath all other layers, just modify the layers element of the plot object.

## For example:
P$layers <- c(geom_boxplot(), P$layers)

---

Answer to the Bonus Question:

This handy little function inserts a layer at a designated z-level:

insertLayer <- function(P, after=0, ...) {
  #  P     : Plot object
  # after  : Position where to insert new layers, relative to existing layers
  #  ...   : additional layers, separated by commas (,) instead of plus sign (+)

      if (after < 0)
        after <- after + length(P$layers)

      if (!length(P$layers))
        P$layers <- list(...)
      else 
        P$layers <- append(P$layers, list(...), after)

      return(P)
    }