Representative R–V_φ plane shown in Fig. 2, with the same rotation curve (red dots) and the Sun’s orbit (cyan dots) along the L_z-level (dashed line). The equidistant levels are used to represent the amplitude of the vertical oscillation of the stellar orbits (see top panel to associate the absolute z_max-values), with the initial conditions z = 0 and V_z = 20 km s⁻¹. Top panel: the amplitude of the vertical oscillation, z_max, calculated along the rotation curve, as a function of R.