I don’t know how anyone can say that graffiti isn’t art. It’s definitely interested in colour and line, it’s definitely a form of expression – its proliferation seems to suggest that it stems from sort of deep human need to express oneself, to leave a mark on a world which otherwise might seem empty and grey. To change something removed and cold, for good or bad, with your own hands. Even in its most basic forms, it’s still a response. You’re still saying something, you’re still speaking and hoping to be heard.
I used to keep a link (on a computer with is now dead and choked with viruses) to translations of roman graffiti found on the walls of the buildings in pompeii. This isn’t it, but it’s got some great examples that are just as interesting. Obviously these are a little different than the pictures I posted above, but you can’t deny our historic need to carve out little places for ourselves on public walls.