I've completed PCA analysis, in R with VEGAN package, of some ecological data on tree health. There are 80 trees total (so, 80 'sites') divided into four treatment categories. I've got the data plotted with color coded points--colors according to the treatment groups. Rather than plotting individual sites/trees on PCA biplot, I'd like to make something like a box-and-whisker plot that has four 'crosses' that show the centroid for each group and the SE in both PCA dimensions. I've seen figures like this in papers, but I can't seem to find an R script for plotting this way. Any suggestions?
I guess an alternative would be to just take the site scores and manually find the means and SE's and create my own plot, but I'd rather find a script for it, if possible.