Relation between maximum HB temperature and metallicity for R06 GCs. The superimposed straight dashed line is a linear least-square fit, and the solid line is a local polynomial regression smoothing. Three points appear visually as outliers to the relation: NGC 7078, NGC 6388, and NGC 6441. The dash-dotted line is the fit obtained without including them in the data-set. The difference with respect to the solid line is relatively small, due to the robust algorithm used for local polynomial regression (see text for details).