Graphic Novels: Literature or not?

Now, I don't only mean manga, much of which is no better written than your usual NYTimes bestseller. I mean also books like Persepolis, Maus, and Sin City, whose style and content make them memorable and worth remembering. I was having a discussion with someone who does not consider graphic novels literature, and it made me rather sad. Good lit is good lit (and rare these days); does it matter if it's written in such a medium? Would Kafka's "Metamorphosis" or Shakespeare's "Hamlet" be any less great if portrayed in graphic novel form?