Density of the age–metallicity relation for the disk stellar sample of each galaxy. The dark blue line corresponds to the projected central metallicity that is used as the anchor point to determine the metallicity profile for the genuine disk (see Section 5). About half the sample shows a dilution in the projected central metallicity, while the other half shows a predominantly [Fe/H]-enriching center. If no blue line is shown, then there were not enough stars to determine the projection.