void trifun (fun, dim, zmin, zstep, zmax, lnstyle, border) TRIANGLE.C Level 2
float (*fun)(float,float) The user-defined function fun(a,b) returning the attribute of the variables associated with vertices A and B. (The vertices are labeled counter clockwise starting with A at the top.)
int dim The number of points to be used to make the grid.
float zmin The minimum value of the attribute to be plotted.
float zstep The step between contours.
float zmax The maximum z value to be plotted.
int *lnstyle An array of coded numbers for changing line styles and/or colors for each contour level. The first 2 digits are 10 + the color number. The third digit is the line style number. 
This specification will not handle the extended GraphiC colors and is retained for compatibility purposes.  If lnstyle == NULL, the more powerful specification capability of contcolor() will be used (recommended).
int border The color of the triangle border and tick marks.
trifun() draws a triangle plot based on data generated by the function fun(). This routine is similar to contour()  
and uses the same internal GraphiC routines. The triangle shape is converted to a right triangle, which then is stretched into a square by pulling the center of the hypotenuse diagonally upward. Contours are drawn in this space and mapped back to the triangle.