Sum of the equivalent widths ΣEW of the two strongest Ca lines plotted over V − V_HB for all 20 clusters in the sample, sorted by mean metallicity. Only stars are included with V − V_HB <  +0.2 as described in the text. A linear fit to the data for each individual cluster is plotted in orange, for which the slope β is given in the title of each panel. The blue lines show the result of a global fit, where the same slope has been used for all clusters, yielding β = −0.581 ± 0.004. In some cases, the individual and global fits are indistinguishable.