The standard BPT  [O   iii] λ5007/Hβ vs.  [N   ii] λ6583/Hα diagram. The 2D histogram of the line ratios for more than 800 000 emission-line galaxies taken from the SDSS DR7 value-added MPA/JHU galaxy catalogue are shown in the grey (logarithmic) scale image. All EELR regions of our sample for which both line ratios could be measured, even if one ratio is just a limit, are plotted as black symbols. Demarcation curves by Kewley et al. (2001), Kauffmann et al. (2003) and Stasińska et al. (2008) are drawn as solid, dashed and dotted blue lines, respectively.