Insert Layer underneath existing layers in ggplot2 object

ggplot2, layer, r

Solution

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)
    }

Problem

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 `geom_boxplot()` to `P` such that the boxplot appears underneath `geom_point()`? ``` ## Starting from: library(ggplot2) P <- ggplot(data=dat, aes(x=id, y=val)) + geom_point() ## This adds boxplot, but obscures some of the points P + geom_boxplot() ``` Expected Output: ``` # Which is essentially ggplot(data=dat, aes(x=id, y=val)) + geom_boxplot() + geom_point() ## However, this involves re-coding all of P (after the point insertion of the new layer). ## which is what I am hoping to avoid. ``` Bonus question: If there are multiple layers in the existing plot, is it possible to indicate where specifically to insert the new layer (with respect to the existing layers)? SAMPLE DATA ``` set.seed(1) N <- 100 id <- c("A", "B") dat <- data.frame(id=sample(id, N, TRUE), val=rnorm(N)) ```

Original source