Hilma af Klint
Her abstract work predates the artist usually credited as its pioneer by several years, yet she deliberately barred public exhibition of it for twenty years after her death — the standard art history timeline and the actual dated record genuinely disagree, and her work was only shown to the public starting in 1986.
Hilma af Klint's Paintings for the Temple, begun 1906; first major public exhibition, 1986
Swedish painter and spiritualist who produced large-scale abstract paintings, guided by group séance sessions, years before abstraction is conventionally said to have begun.