Cluster metallicity as a function of Galactocentric distance computed from the coordinates of the clusters and their distance from the Sun taken from the Dias et al. (2002, version 3.2) catalogue. The distance of the Sun from the Galactic centre (at the right of the graph) is assumed to be 8 kpc. Open symbols are clusters with only one metallicity measurement. Also shown are predictions of several models for the Galactic chemical evolution. See Sect. 5.2 for details.