A couple of days ago I mentioned Louis Wain, the British artist who became famed as a cat painter - and sufferer of schizophrenia. There's two schools of thought on his art, I've just found: first, that his illness radically changed his approach to his subject matter; second, that he developed artistically as he would in any case, and the illness was secondary.
Even leaving aside the illness and institutionalization, his life was sad much of the time. Yet from his struggles he pulled what became some of the most engaging cat art of his time, or ever. All this is said and illustrated beautifully at the site
Be sure and read the interview with him (taken from a publication of the time). Note the part where the interviewer remembers watching five poor children, cold and hungry, get cheered up by the sight of a Wain cartoon in a barbershop window. Oh! my heart.