Hi Chris,
I've tried using this example in one of my plots, and I find that the point appears at the coordinate (1,7) rather than (30,7). The coding that works is:
b: -0.4*x+19;
ptx:[30];
pty:[7];
{@plot([[discrete,ptx,pty], b],[x,0,60],[y,0,20],[style, points, lines], [color, red, blue],[point_type, asterisk])@}
Paul