Of the 10 artists in the ranking, Ibrahim El-Salahi is the only one known to have done time in jail - and not just any jail; a squalid cell in Khartoum, the Sudanese capital, which he shared with nine other people during six months of detention in the mid-1970s. Somehow, despite the serious risks involved, he kept making art and drawings in prison that he kept hidden away. (El-Salahi, who was then the country’s undersecretary for culture, has been detained without charge because a cousin of his had attempted a coup; he was later released without explanation.) The son of a Koranic scholar born in 1930, he became an art student and secured a scholarship to London’s prestigious Slade School of Art in the 1950s. By the late 1950s, he was heading back to Sudan, and embarking on a career as an artist, though his works went largely unnoticed, both in his own country and internationally. Since 1998, he has lived in Britain, which has considerably raised his international profile. In 2013, El-Salahi became the first African artist to get a solo exhibition at Tate Modern. He presented dozens of his Surrealist-like paintings, and a few of the drawings he produced immediately after his release from prison.