Distribution of the low-flux and high-flux samples in the redshift-flux plane and in the BPT diagram. Top panel: two samples of radio galaxies in the redshift-flux density plane: a high-flux sample of 119 sources previously analysed by Vitale et al. (2015a) and a sample extended towards lower radio flux densities at 1.4 GHz, 10 mJy ≤ F_1.4 ≤ 100 mJy (black points). The grey points represent a subset of the parent sample in the redshift range of 0.04 ≤ z ≤ 0.4. Bottom panel: selected radio galaxies for the Effelsberg observations in the [NII]-based BPT diagnostic diagram (black points). The sample of radio galaxies with F_1.4 ≥ 100 mJy analysed by Vitale et al. (2015a) is denoted by green crosses. The parent sample is colour coded depending on the density per bin in the [OIII]–[NII] plane (bin size is set to 0.1 along both ratios).