R has an external package called VCD which should do what you want.
The documentation is very good (122 page manual distributed w/ the package); there's also a book by the same name, Visual Display of Quantitative Information, by the package's author (Prof. Michael Friendly).
To create ternary plots using vcd, just call ternaryplot() and pass in an m x 3 matrix, i.e., a matrix with three columns.
The method signature is very simple; only a single parameter (the m x 3 data matrix) is required; and all of the keyword parameters relate to the plot's aesthetics, except for scale, which when set to 1, normalizes the data column-wise.
To plot data points on the ternary plot, the coordinates for a given point are calculated as the gravity center of mass points in which each feature value comprising the data matrix is a separate weight, hence the coordinates of a point V(a, b, c) are
V(b, c/2, c * (3^.5)/2
To generate the diagram below, i just created some fake data to represent four different chemical mixtures, each comprised of varying fractions of three substances (x, y, z). I scaled the input (so x + y + z = 1) but the function will do it for you if you pass in a value for its 'scale' parameter (in fact, the default is 1, which i believe is what your question requires). I used different colors & symbols to represent the four data points, but you can also just use a single color/symbol and label each point (via the 'id' argument).
