Top panels: radial (top left) and vertical (top right) metallicity distributions of our sample of 597 OCs, colour-coded by age. 1000 MC samplings (thin grey lines) in each panel are superimposed; the red and blue solid lines are the best fitting lines. The red or blue shaded bands in each panel corresponds to the 1σ dispersion around the gradient. The knee positions of red and blue breaks in the top left panel are 10.80±0.04 kpc and 12.43±0.04 kpc, respectively. The three outliers are indicated. Bottom panels: posterior distributions of the fit parameters of radial (bottom left) and vertical (bottom right) metallicity distributions.