An equation of the form y=ax+b, where y and x are variables, and a and b are constants, is called a linear equation.
The reason it is called linear is because the graph of the equation is a line.
The line passes through the origin only if b=0, as in our problem.
Any line (except vertical lines) represents a proportional relationship in that the change in y is proportional with the change in x. It is not a condition that the line passes through the origin.
In our specific case, y=(1/5)x, means that the change in y is always 1/5 of the change in x. That is, if x changes by 5 units, y changes by 1. If x changes by 10 units, y changes by 2, and so on.
So, the constant of proportionality is 1:5, or 0.2.
