A linear function has the form y=mx+b
A line has a proportional relationship if y/x is always the same ratio for any value.
The slope m=(y2-y1)/(x2-x1) for some two points on a line is always constant, else it wouldn't create a line.
A line won't be proportional if you adapt b because the ratio of y/x won't match the slope anymore.
In the end this means all lines with proportional relationships must intersect (0,0) or in other words f(0)=0.
This happens if they have the shape y=mx.
