Examples of the fits to the [Fe/H] vs. R_Gal distributions for three age bins (from top to bottom: 0.5-1 Gyr, 4.5-5 Gyr, 10.5-11 Gyr). Left panels: distribution of the red-giant stars in each age bin. The thick black line shows the result of a naïve least-squares linear fit. The thin grey lines show 30 realisations drawn from the linear gradient + intrinsic scatter posterior, while the shaded band corresponds the 1σ dispersion around the gradient. Right panels: posterior distributions of the fit parameters (m = ∂[Fe/H]/∂R, b (intercept at R = 0), and σ (intrinsic [Fe/H] dispersion).