The english unit of force equal to 4.448 newtons

That's the "pound-force".

There's another English unit called the "pound-mass".
It's the mass that weighs 1 pound-force ON EARTH.

Now you can appreciate how the SI system can actually be LESS-confusing.  

Alinear relationship would look like a straight line on the graph. it could have a positive negative or 0 slope. so it could be a line potentially going in any direction but it must be perfectly straight.