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To determine experimentally whether two physical quantities are directly proportional, one performs several measurements and plots the resulting data points in a Cartesian coordinate system.If the points lie on or close to a straight line that passes through the origin (0, 0), then the two variables are probably proportional, with the proportionality constant given by the line's slope.

I've been told by people that a graph with a straight line on it can be proportional but only if it passes through the origin. I fail to see why that's true. If a translation was applied and it was moved 1 unit to the right then, all of a sudden, x is not proportional to y anymore? That doesn't make sense to me.