The structure of multicomponent fluids in confined geometries is a key to understanding their properties. However, it remains an experimental challenge to gain molecular-scale resolution information on this structure. Here we show that mono- and multilayers of graphene, conforming to heterogeneous monolayers of molecules in a flexible slit pore between a mica surface and the graphene layers, allow for mapping the phase separation of water and ethanol within such a slit pore. Employing scanning force microscopy, we readily distinguish clusters of ethanol and water molecules due their different sizes, and we show that the phase separated water ethanol structures become coarser under thicker graphenes. Moreover, we obtain a lower bound for the two-dimensional diffusion coefficient of ethanol in water of D >= 2 x 10(-14) m(2) s(-1). Thus, the molecularly thin slit pore provides a powerful tool to control and to investigate mixed fluids in self-adjusting nanopores.