[O iii] λ5007/Hβ vs. [N ii] λ6583/Hα diagnostic diagram for the ~5000 H ii regions described in the text. The contours show the density distribution of these regions with the diagram plane, with the outermost contour enclosing 95% of the regions and each consecutive one enclosing 20% fewer regions. In each panel, H ii regions corresponding to different galaxies with different morphological types are represented: (i) E/S0/Sa top left; (ii) Sb top right; (iii) Sbc bottom left; and (iv) Sc/Sd/Irr bottom right panel. In all panels, the dot-dashed and dashed lines represent the demarcation curves described in Fig. 1.