Relation between the halo radio power at 1.4 GHz and the cluster X-ray luminosity between 0.1 and 2.4 keV. A1689 is indicated with a red triangle. Blue dots are classical radio halos, while empty blue dots are outlier clusters. The data are taken from Giovannini et al. (2009) and references therein, with the addition of CL0016+16 (Giovannini & Feretti 2000), MCSJ0717+3745 (Bonafede et al. 2009), A399 (Murgia et al. 2010), A781 (Govoni et al. 2011), A523 (Giovannini et al. 2011), and 0217+70 (Brown et al. 2011).