Hydrogenography optically monitors hydrogen ab- and desorption simultaneously on thousands of alloy compositions under exactly the same experimental conditions. In this way the hydride-formation enthalpy map of the Mg-Ni-Ti-H system is determined from a compositional gradient thin film. The figure shows that the region of interesting hydrogen-storage materials is in dark blue, with enthalpies of around -40 kJ (mol H-2)(-1).