You can only plot a triangular graph if a+b+c=100%
So you can create a triangular graph using an X-Y scatter by calculation -you can plot points using any two of a b and c as the third is fixed by the other two. You need to do a bit of fiddling to get it looking right though.
Depending on what you want the top corner of your triangle to be you set that as the Y value for your series (lets assume it is b and a is at 0,0 and c is the right hand side of the base)
You then calculate the X value as c+1/2b for each of your points (simple trigonometry) and plot your points in teh normal way for an X-Y scatter
Set your formats for the X and Y axes so they are fixed - minimum 0 maximum 100 and set the formats to make all the lines disappear
You then need to add 3 data series to provide the sides of your triangle - one from 0,0 to 50,100 and one from 50,100 to 100,0 and a third from 0,0 to 100,0
If you want to put in lines to provide a scale of grid lines to break up the triangle - say showing every 10% you need to calculate them using teh same formula and plot them as series too (so the line for 10% B which is parallel to the base plots from 5,10 (10% b and 90% a)to 95,10 (10%b and 90% c) and so on). Fiddle with the formatting of teh data series so it looks good and teh job is done
