Start image: The Joy Division album cover. Former Factory Records graphic designer Peter Saville designed Joy Division's Unknown Pleasures album cover after seeing a stacked plot of radio signals from a pulsar in The Cambridge Encyclopaedia of Astronomy. But where did the stacked plot come from? In 2015, Scientific American's senior graphics editor Jen Christiansen traced it back to the work in 1970 of PhD student Harold D Craft Jr at the Arecibo Radio Observatory, Puerto Rico. After hearing of Jocelyn Bell Burnell's discovery of the pulsar CP1919 on 28 November 1967, Harold had decided to plot it 80 successive times – and an iconic image was born.