Gravitational potential, Φ(400 pc)−Φ(0 pc), as a function of galactocentric radius, for the four stellar samples (as labelled in the panels’ top-right corner). The dashed line is the exponential function of Eq. (9) fitted to data points with $\sqrt{X^2+Y^2}<2\mathrm{kpc}$, which excludes distant and dubious spatial regions. The scatter points correspond to area cells in the (X, Y) plane. Area cells that are included in the exponential function fit are coloured according to their galactocentric disk plane angle (ϕ); excluded data cells have light grey and smaller scatter points. The colour bar and axis ranges are shared between all panels.