The constant of proportionality is always the where k is the constant of proportionality

Let's take as example a linear function of the form:
 y = kx.
 Where,
 k is the constant of proportionality.
 Therefore, the proportionality constant is the point:
 (x, kx)
 Generically it is always the point:
 (x, k * f (x))
 Where, f (x) is a function proportional to x.
 
 The constant of proportionality is always the point (x, k * f (x)), where k is the constant of proportionality.