The graph isn't just a divisibility test, it reports the remainder after division by seven. I specifically acknowledged that this error won't mutate a "divisible by 7" result into "not divisible by 7" (because any number that is divisible by seven will still be divisible by seven after multiplying it by ten), but in every other case it will give you the wrong answer. (e.g. "remainder 2" will mutate into "remainder 6".)