[O iii] λ5007/Hβ vs. [N ii] λ6583/Hα diagnostic diagram for the ~5000 H ii regions included in our sample. The contours show the density distribution of these regions within the diagram plane, with the outermost contour enclosing 95% of the regions, and each consecutive one enclosing 20% fewer regions. In each panel the color indicates the average value at the corresponding location in the diagram of the luminosity-weighted age (left panel) and metallicity (right panel) of the underlying stellar population, respectively. The gray solid lines represent the expected line ratios derived from the photoionization models shown in Fig. 1, where each line corresponds to a different stellar metallicity (orange-to-red solid lines in Fig. 1). In both panels, the dot-dashed and dashed lines represent the demarcation curves described in Fig. 1.