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 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.  

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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)?

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SAMPLE DATA

set.seed(1) N <- 100 id <- c("A", "B") dat <- data.frame(id=sample(id, N, TRUE), val=rnorm(N)) 

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) 

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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

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.