Observational HRD with the mean value of A_proxy,G from Subset Bgp (left panel) and of A_proxy,BP/A_proxy,RP from Subset Cgp (right panel) for each cell of size [Δ(G_BP − G_RP),ΔM_G] = (0.045, 0.12) mag, plotted in colour according to the colour-scale on the right of each panel. The thin contour lines in black correspond to the density lines of the DR2 sample of constants+variables shown in Fig. 22 (bottom panel). The thick lines correspond to evolutionary tracks of (from bottom to top) 1, 2, and 5 M_⊙ solar-metallicity stellar models from Ekström et al. (2012).