My WordWeb defines 'graffiti' as "a rude decoration inscribed on rocks or walls" and the Merriam-Webster has it down as "usually unauthorised writing or drawing on a public surface."
These definitions are archaic, I think. Graffiti can be authorised — like the Wall Project by the BMC in Bombay, which you can read about on the Indian Express and DNA website. Graffiti can also be considered an alternate art form, a tool to express one's views in a visual way, on a completely different canvas. Idea-Smithy has photographed some of their graffiti along the Mahim–Matunga railway station wall.
The graffiti I came across was orchestrated on a 'different' canvas alright. A currency note. And it was rude. Not very, actually. Since we live in a city where we hear far worse words than "Motu, Gandu, Saala". I do not think the Books o' Law have anything on the act of writing on Indian currency notes. So there. Graffiti in its various forms. This one, on a ten rupee note. The Father of our Nation still manages to hold that smile while staring at those words..