Now you've nailed the semantic(?) problem. WHAT line, WHERE?
The one from (x1) to (x2)?
Or the one from (x1,y1) to (x2,y2)?
Or the one from (x1,y1,z1) to (x2,y2,z2)?
The first two don't tell me enough if in 3D space. (?)
If I'm frustrating anyone, please tell me. I'm still not...