I read the thread started by epitaph about ternary plots, but I am stymied by the R documentation available for all the ternary plotting routines. I'm trying to plot pyroxene compositions, where I'm plotting Mg vs Fe vs Ca and I need to draw a single gridline at 50% Ca component. I can't find any way to change the tick/gridline positions at all for any of the routines mentioned in the previous thread. Can anyone help me? I'm happy to use something other than R...I only started using it specifically for this.