Adding symbols to dendrogram leaves in R

Question

I would like to add symbols to the leaves of a dendrogram reflecting variable for sites on a dendrogram similar to this:

require(graphics)

hc <- hclust(dist(USArrests[1:5,]), "ave")
plot(hc)
plot(hc, hang = -1)

USArrests[1:5,]

           Murder Assault UrbanPop Rape
Alabama      13.2     236       58 21.2
Alaska       10.0     263       48 44.5
Arizona       8.1     294       80 31.0
Arkansas      8.8     190       50 19.5
California    9.0     276       91 40.6    

Thanks for any suggestion on how to go about this

SOLUTION Following Backlin's helpful suggestions I use the following solution

require(graphics)

hc <- hclust(dist(USArrests[1:5,]), "ave")

plot(hc, hang = -1, xlab="", sub="")

col.circle=c("yellow", "red")[cut(USArrests$Murder[hc$order], c(8,10,15))]

symbols(1:5, rep(-25, 5), circles=rep(1, 5), add=TRUE, inches=.2,bg=col.circle, xpd=TRUE)

col.square=c("blue", "green")[cut(USArrests$Assault[hc$order], c(100,200,300))]

symbols(1:5, rep(-35, 5), squares=rep(1, 5), add=TRUE, inches=.4,bg=col.square, xpd=TRUE)

legend(3.7,85,legend=c("Murder 8-10","Murder 10-15","Assualt 100-200","Assualt 200-300"),fill=c("yellow","red","blue","green"))

Answer 1

The symbols function could be used to accomplish this in a somewhat roundabout, but still effective way. It does not support triangles, but has a few other shapes to choose from. In the demonstration below, note the parameter xpd=TRUE that allows you to draw outside the plot area, i.e. in the margin.

plot(hc, hang = -1, xlab="", sub="")
symbols(1:5, rep(-25, 5), circles=rep(1, 5), add=TRUE, inches=.2,
        bg=rep(c("grey", "red"), c(3,2)), xpd=TRUE)
symbols(1:5, rep(-35, 5), squares=rep(1, 5), add=TRUE, inches=.4,
        bg=rep(c("grey", "red"), c(1,4)), xpd=TRUE)

For readability of the example, the y coordinates of the symbols are set to absolute values. If you want to make them relative to the coordinates of the plot use something like the following, where par("usr") is a vector with (x-left, x-right, y-bottom, y-top) of the plot area.

y = par("usr")[3] - .04 * diff(par("usr")[3:4])

The legend can also be plotted with symbol and text. The idea is the same and you can probably figure it out yourself, even though it is quite fiddly.