2D age-metallicity probability map, here called a 𝒢 function, calculated for the example star in Sect. 2.3, using the Gaia G band, and the colours (G − G_BP) and (G − G_RP). The colour bar shows the log of the marginalised likelihood, normalised to 1 at its highest point, with the 90% confidence interval highlighted with a solid dark line. The red circle represents the point of highest probability, and the red cross represents the star’s true [Fe/H] and age.