Best-fitting lines from Table 2 for which the relative errors on the two parameters a and b are lower than 20%. Solid lines represent the best-fitting relations for each sample, listed in Table 2, while their corresponding confidence intervals are shown by the shaded bands. The left panels show results for the NVSS cutouts, and the right panels show the results for the FIRST cutouts. Detections are marked with points, and arrows are used to show nondetections, both with accompanying error bars on both axes.