This makes me laugh, you all know modern day graffiti pre dates the hip hop movment.
When they were painting trains in the 70's hip hop wasn't widley recorded and most writers listened to funk/rocknroll/metal and some were even punks. The writers and cholos on the west coast bumped oldies from the 40's and 50's and electro.
Graffiti got put in with hip hop beacause artists were doing the fliers for all the jams. Case Closed!