Distribution of the radio luminosity at 1.4 GHz, L_1.4 GHz, in optical diagnostic diagrams. Top row: distribution of the radio luminosity L_1.4 GHz for the parent sample. The contours correspond to the distribution of the sources for the luminosity bins, log(L_1.4 GHz) <  40.5, 40.5 <  log(L_1.4 GHz) <  41.2, and log(L_1.4 GHz) >  41.2, with the increasing radio luminosity from left to right. Middle row: distribution of the radio luminosity L_1.4 GHz as in the top row but for a flux-limited subsample of the parent sample, 10 mJy ≤ _1.4 GHz ≤ 1000 mJy. The luminosity bins are the same as in the top row. Bottom row: distribution of the radio luminosity L_1.4 GHz for the Effelsberg sample only (low + high flux sources). The luminosity bins are the same as in the top row.