Diagnostic diagrams containing the results of our photoionization models and observed data. a) [OIII]λλ5007/ Hβ vs. [NII]λλ6584/Hα and b) [OIII]λλ5007/Hβ vs. [NII]λ6584/ [OII]λλ3726, 3729. In all the diagrams, the dotted lines connect points with the same ionization parameter. The dashed lines connect curves of equal log (N/O). The respective values are indicated at the beginning of curves. For all the models we assumed 12 + log (O/H)) = 8.47. Points are values of integrated spectra placed according to their intensities ratios (named as in Table 3 and Fig. 9).