[R] x/y coordinates of dendrogram branches

[Timo Becker]

Dear R-users,

I need some help concerning the plotting of dendrograms for hierarchical 
agglomerative clustering.
The agglomeration niveau of each step should be displayed at the 
branches of the dendrogram.
For this I need the x/y coordinates of the branch-agglomerations of the 
dendrogram.
The y-values are known (the heights of the agglomeration), but how can I 
get the x-values?

 > mydata <- c(1,2,3,4,5)
 > hc <- hclust(dist(mydata), method="average")
 > hc$height # these are the y-coordinates
[1] 1.0 1.0 1.5 2.5

I experimented with the dendrogram object because it gives the midpoints:

 > dend <- as.dendrogram(hc)
 > attributes(dend[[1]])$midpoint
[1] 0.5

Perhaps I could loop over all possible branches and get the midpoints if 
I knew how to convert them to x-values.
The manual says that midpoint is the "numeric horizontal distance of the 
node from the left border (the leftmost leaf) of the
branch (unit 1 between all leaves)."
But how do I know which one is the leftmost leaf and what is its 
horizontal value?

Thanks in advance,
Timo