R help - Aug 2006 - centroid of manually given groups in a cca-plot

dear R-friends,

i have a set of vegetation and environmental data:

	veg <- read.table("http://www.alectoria.at/dl/rmalliveg.csv",
header=TRUE, sep=";")
	env <- read.table("http://www.alectoria.at/dl/rmallienv.csv",
header=TRUE, sep=";")

with the library "vegan" i did a constrained correspondence
analysis(cca):

	rmcca <- cca(veg,env)

and with ordihull from "vegan" it is possible to draw a line or
polygon around manually
given group(s) - (groups = Factor giving the groups for which the graphical item
is drawn) - in the plot of the cca.

	rmlab <- read.table("http://www.alectoria.at/dl/rmlabel.csv")
	attach(rmlab)
	ordihull(rmcca, V1, display="sites")

my question in a (more general) way: is it may be possible to get the
centroid-coordinates from such
manually given groups of a cca or is it may be possible to draw the centroid of
such group(s)?

with greetings from austria
helli
> version
               _                         
platform       i386-pc-mingw32           
arch           i386                      
os             mingw32                   
system         i386, mingw32             
status                                   
major          2                         
minor          3.1                       
year           2006                      
month          06                        
day            01                        
svn rev        38247                     
language       R                         
version.string Version 2.3.1 (2006-06-01)