A proportional relationship is described by the equation
... y = k·x
The point (x, y) = (0, 0) is always a solution to this equation.
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In short, if the relationship is proportional, its graph will go through the origin. If the graph does not go through the origin, the relationship is not proportional.
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Note that this is true if the domain includes the origin. You can have y = kx for x > 10 and the graph will not go through the origin because the function is not defined there.