Upper panel: the relation between σ_Pθ and M_G for supergiants and bright giants, assuming either an exponential (red plusses) or a parabola (blue crosses) to connect the box-in-a-star with the star-in-a-box models (Fig. 18). The equation of the dashed green line (through the red plusses) is , whereas the equation of the magenta dotted line (through the blue crosses) is . Lower panel: same as the upper panel, for the relation between the pressure scale height Hp and the absolute magnitude in the Gaia G band. Only stars with a detectable photocentric motion (for the exponential fit: red crosses; for the parabolic fit: green dot) have been plotted.