For arbitrarily-angled lines, the polygon writer utility comes to mind. I'll look into it when I finish this bath.
I completely forgot about this after I finished my bath. The polygon writer utility let me down. But whatever, I was working from false assumptions anyway. Either way, (Graph grDRAW_LINE y1 x1 y2 x2 [visualColor priColor ctlColor]) can be used to render the actual line.