a) Relation between the electron temperatures t_e(Mg ii) and t_e(O ii). t_e are in units of 10^-4   T_e. Red filled circles, blue open circles, and green stars are data from CLOUDY models with starburst ages of 2.0 Myr, 3.5 Myr, and 4.0 Myr, respectively. The solid line is the fit to data with all ages, defined by Eq. (2). The dotted line is the line of equal temperatures. The range of the electron temperature t_e(O ii) in the sample galaxies is shown by a horizontal line. The error bar is the dispersion of t_e(Mg ii) in the range of t_e(O ii) shown by the vertical line. b) Relation between the ionisation correction factor ICF(Mg⁺) and the excitation parameter x = O²⁺/O. Red filled circles, blue open circles, and green stars are data from CLOUDY models with starburst ages of 2.0 Myr, 3.5 Myr, and 4.0 Myr, respectively. The solid line is the fit to data with all ages as defined by Eq. (5) with the coefficients from Table 3. The range of the excitation parameter x in the sample galaxies is shown by a horizontal line. The error bar is the dispersion of ICF(Mg⁺) (in dex) in the range of x shown by the vertical line.