Ternary Plots with continuous data

Hello 

I have a data base consisting of soil parameters, and tree species 
densities.   The vcd (visualizing categorical data) package includes the 
ternaryplot function which plots the gravitation center of 3 prameters. 

I'd like to find a similar function for use with continuous data rather than 
categorical. 

Does anyone know of a function suitable for this objective? 

Thanks 

Steve Friedman