Plot of an equilateral triangle, in order to depict trinomial
distributions. A trinomial distribution (that is, a trio of
non-negative numbers that add to 1) is equated to a point in the
triangle through the distances to the three sides. This makes use of
the fact that for any point in an equilateral triangle, the sum of the
distances to the three sides is constant.
First use triplot() to first plot the equilateral triangle.