The Stata program triplot produces a triangular plot of three variables with constant sum. Most commonly, three fractions or proportions add to 1, or three percents add to 100. The constant sum constraint means that there are just two independent pieces of information. Hence, it is possible to plot observations in two dimensions within a triangle, which is a 2-simplex. Triangular plots appear under various names in the literature, including trilinear, triaxial, three-element maps, ternary, reference triangles, percentage triangles, mixture, barycentric. Common geological applications are to sedimentary facies or particle form; hence, more specific terms such as facies and form triangles. triplot has several options for tuning displays. A variety of data examples will be shown to indicate some of the flexibility of the program.
