What is the theory which states that the relationship is the cover graph is linear? There must be one otherwise the fit does not make sense, it could have been

What is the theory which states that the relationship is the cover graph is linear? There must be one otherwise the fit does not make sense, it could have been ax^3 + e^9x, or heart shaped. There should be a law punishing arbitrary linear fits and ridiculous claims which are concluded from their correlation.