Add a 'shadow' visual effect to a plotted tree in R

Today I saw a cool visual effect in a plotted phytools object, although I suspect it was achieved by posthoc editing the plot in Adobe Illustrator rather than in R.

The effect was to add a 'shadow' to a plotted tree. Here is a quick demo of how to do it in R. I have illustrated the method using a plotted "contMap" object, because it is a little trickier, but we could do the same thing with any old plotted tree:

## load phytools
library(phytools)
## here is our tree & data
tree

## 
## Phylogenetic tree with 26 tips and 25 internal nodes.
## 
## Tip labels:
##  A, B, C, D, E, F, ...
## 
## Rooted; includes branch lengths.

x

##           A           B           C           D           E           F 
## -1.28663899 -2.18638418 -1.76955549 -1.85871833  0.92086981 -2.31821640 
##           G           H           I           J           K           L 
##  3.04198683  1.41705375  1.35059487 -0.03275257  0.19923947  0.09906219 
##           M           N           O           P           Q           R 
##  0.84486447  0.06387185  0.27210453  1.02166697  0.85217792 -0.31925792 
##           S           T           U           V           W           X 
## -1.17999809  2.67084163  3.05705763  2.77059737  7.84958715  5.18219507 
##           Y           Z 
##  3.78072083  4.69439548

## get our "contMap" object
obj<-contMap(tree,x,plot=FALSE,res=1000)
## now this is really just a test plot that we are going to
## compute to get our x & y limits
plot(obj,lwd=6)

lastPP<-get("last_plot.phylo",envir=.PlotPhyloEnv)
## now our real plot:
par(fg="transparent")
plotTree(obj$tree,color="darkgrey",lwd=8,xlim=lastPP$x.lim,ylim=lastPP$y.lim)
xlim<-lastPP$x.lim-0.004*diff(lastPP$x.lim)
ylim<-lastPP$y.lim-0.004*diff(lastPP$y.lim)
## this is not for the shadow, but to get an outline on our
## plotted contMap tree:
plotTree(obj$tree,lwd=8,xlim=xlim,ylim=ylim,add=TRUE)
par(fg="black")
plot(obj$tree,colors=obj$cols,lwd=6,add=TRUE,xlim=xlim,ylim=ylim)
## finally, add our legend, also with a shadow
lines(c(0,0.5*max(nodeHeights(obj$tree)))-0.004*diff(lastPP$x.lim),
    c(-1,-1)-0.004*diff(lastPP$y.lim),lwd=8,lend=2,col="darkgrey")
add.color.bar(leg=0.5*max(nodeHeights(obj$tree)),cols=obj$cols,
    title="trait value",prompt=FALSE,x=0,y=-1,lims=obj$lims,lwd=6)

Pretty neat - I think.

The tree & data for this example were simulated as follows:

tree<-pbtree(n=26,tip.label=LETTERS)
x<-fastBM(tree)