Shape of radial [Fe/H] gradient. Upper panel: [Fe/H] vs R_GC of our sample clusters. The blue line is a weighted linear regression model fit to the global trend and the shaded area shows the confidence interval. The cyan curve is the lowess fit. Bottom panel: [Fe/H] vs R_GC with weighted linear regression model fits in two radial ranges, R_GC ≤ 11.2 kpc (in brown) and R_GC >  11.2 kpc (in turquoise). The shaded areas show the confidence intervals of the two fits and the dashed grey curves are the lowess models in the two intervals. The vertical grey line indicates the slope change point calculated by the elbow method. The dashed vertical and horizontal lines mark the location and abundance of the Sun.