Art or Not?

There is a documentary about graffiti and the hip hop culture which I can't remember the title to that I had watched recently. According to the film, graffiti originated with individuals that "tagged" their signs on railroad carts so that many people could see them as they rolled  throughout the country from state to state. One of the more famous international tags was, "Kilroy was here". Other individuals would paint over the tags to show off their painting skills as gang rivalries. Graffiti was meant to be temporary as they faded with time or was painted over by others, until some rich guys decided to buy and sell them as art on canvasses and to hang them in museums and indoor spaces. In Hawaii, the graffiti are usually just scribbles although I can't take anything away from self expression. My question is, is it art or not?