I assume you don't know what the original joke is? The original is, in fact, a graph, this is a joke from a movie called Crocodile Dundee, where a guy pulls a knife on the main character, who proceeds to go "that's not a knife" and pull out the largest bladed weapon that could still be considered a knife and says "this, is a knife."

Hope this was helpful and that I'm not explaining something you already know.