O3Hβ–S2Hα diagram of the LyC leakers (open circles) and comparison sample with metallicities 12 + log(O/H) = 7.6 − 8.2 (gray dots). The plain blue line shows the ridge line of our SDSS sample, and the black dashed line is the ridge line from Wang et al. (2019) extrapolated to high [O III]/Hβ values. Many leakers, especially the strongest ones, show a deficiency in [S II] with respect to this latter ridge line.