Left panel: distribution of the [N ii] λ6583/Hα line ratio along the luminosity-weighted ages of the underlying stellar population for the ~5000 H ii regions included in our sample (individual dots), with a color code representing the luminosity weighted metallicity of the underlying stellar population. The solid circles and individual lines illustrate how the considered ratio change along the stellar population age for a certain metallicity range. Each circle represents the average line ratio within a bin of 0.25 dex in log(age/yr) for a fixed range of metallicities (within a range of 0.1 dex). Consecutive circles for a particular metallicity are linked with a solid line, and both of them are represented by a fixed color, different for each metallicity. The actual value of the metallicity corresponding to each color is indicated in the enhanced box. Right panel: similar distribution for the [O iii] λ5007/Hβ line. Symbols, solid lines, and color codes are the same as presented in the previous panel.