Found this
http://sul-server-2.stanford.edu/byauth/burke/solpar/solpar7.html
through Google. Can't comment on how the parameters are derived, but
the plot appears to be a pretty standard ternary diagram. Excel
doesn't have the capability built into it to rotate the Y axis (some
other plotting programs do); Excel insists that the Y axis should be 90
degrees relative to the X axis. I occasionally need to plot ternary
diagrams, and either use a right triangle or hide the y-axis and plot
my own axes. I can't remember the transformation off hand, but the
formulas for transforming square cartesian coordinates to the
triangular coordinates aren't difficult to derive-- it's pretty simple
geometry/trigonometry.
If you want, a google search for ternary plot in excel yields several
hits that claim to have worksheets for drawing ternary diagrams.
