Top panel: “Corner plot” (Foreman-Mackey et al. 2016) showing an example of an MCMC fit result for the linear gradient + variable scatter model to CoRoGEE data, in the age bin 6 Gyr <τ< 10 Gyr. Histograms show the marginal posterior PDFs for the fit parameters (m,b,m_σ,b_σ); the density-scatter plots show the joint marginal posteriors. Bottom panel: resulting fit to the data in the R_Gal vs. [Fe/H] diagram. Faint grey lines show 50 MCMC samples; the thick blue line and the shaded band correspond to our best parameter estimates. For comparison, we also show the results of a least-squares linear fit (dotted line) and a maximum-likelihood fit (dashed line).