Take a historian of eighteenth-century art, an expert in the aesthetics of balance, harmony and taste, and send him to make a film about spray-can scrawlings, satirical stencils, and ugly marks scratched on walls. That was the starting point for A Brief History of Graffiti (26 August).Dr Richard Clay started with the marks – mostly names, with the odd triumphant slogan – left by the Soviet soldiers who captured the Reichstag at the end of the Second World War, now preserved as a rather moving historical monument. He suggested that a compulsion to leave a mark that says “I was here” is at the heart of the impulse to leave graffiti.That explanation made a sort of sense while he stayed deep in history. It certainly covered the stencilled image of a hand he found in th