[R] coloring leaves in a hclust or dendrogram plot

[Dylan Beaudette]

Greetings,

I have perused the r-help mailing list archives for an answer to this 
question, without avail. 

I would like to color the "leaves" of a dendrogram plot based on a cutoff in 
one of the variables involved in the initial clustering.

My input data is in the form of:
                  B         K
Alameda   0.2475770 0.7524230
Alpine    0.4546784 0.5453216
Amador    0.6278610 0.3721390

essentially rows labeled by county name, with two variables: percent voted for 
B and percent voted for K. While it is obvious that this is somewhat of a 
contrived example, I intend to use this as a learning device.

Here is the code used to create and plot the dendrogram:
hc <- hclust(dist(y), "ave")
dend <- as.dendrogram(hc)
plot(dend, main="CA 2004 Election Results by County")

An example of the output can be found here:
http://casoilresource.lawr.ucdavis.edu/drupal/node/206?size=_original


I have experimented with the edgePar and nodePar parameters for the 
plot.dendrogram() method, but have not been able to make sense of the output. 

The basis for setting the colors of the leaves in the dendrogram is a simple 
majority calculation:

reds <- y[y$B > 0.5, ]
blues <- y[y$K > 0.5, ]

Such that leaves in the tree will be colored based on the membership in either 
of the two above groups.

Is there a resource documenting how this might be accomplished? 

Any thoughts or ideas would be greatly appreciated.

Cheers,

Dylan






-- 
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341