Insert Layer underneath existing layers in ggplot2 object

Question

Given an Existing plot object is it possible to add a layer UNDERNEATH an existing layer?

Example, in the graph below, is it possible to add

Answer 1

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)
    }

Answer 2

As shown in the ggplot documentation here, you can set up a skeleton ggplot object and add layers. The order in which you specify the layers is the order in which they will appear on your plot.

This will get your expected output:

ggplot() +
  geom_boxplot(data = dat, aes(x=id, y=val)) +
  geom_point(data = dat, aes(x=id, y=val)) 

This will get your incorrect output:

ggplot() +
  geom_point(data = dat, aes(x=id, y=val)) +
  geom_boxplot(data = dat, aes(x=id, y=val)) 

I think this answers your bonus question too :)

Answer 3

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.