I currently have:

plot3d(78.9*x+49.1*y+15.7*(1-x-y), x = 0 .. 1, y = 0 .. 1)

I have ratios x, y and z and the ratio of z is implicit from the ratios of x and y (z=1-x-y), therefore whenever x+y>1 the surface is meaningless as it gives z as a negative ratio.

I want the surface to be plotted only for x+y<1, so something like:

plot3d(78.9*x+49.1*y+15.7*(1-x-y), x = 0 .. 1, y = 0 .. 1, x+y<1)

But I can't find a suitable command.

Thanks in advance,

Tom.