I searched through the inter-web to try and find the oldest photo of graffiti. I found this from the Victorian State Library records.
I came up with this pic of Box Hill from 1977. Graffiti has changed a lot since then. This photo shows graffiti as a dialogue between the writer and society. This wall faces the train line and would be seen by the passing train audience. The political graffiti is indicative of the time it was painted during a period of political awareness and progressive thoughts and ideas.