[S iv]/[Ne ii] vs. [Ne iii]/[Ne ii] line flux ratio plots for the GOALS sample compared with an instantaneous starburst model assuming an electron density of n _e = 10 ² cm ⁻³. The age evolution of the line ratios is shown from the top left to the bottom right panels in 0.5 Myr steps. The parameters of the model are age (indicated in each panel), metallicity (dark blue: Z = 0.4  Z _☉, light blue: Z =  Z _☉, green: Z = 2  Z _☉), and ionization parameter (orange: q = 2 × 10 ⁷ cm s ⁻¹, magenta: q = 8 × 10 ⁷ cm s ⁻¹, red: q = 4 × 10 ⁸ cm s ⁻¹). The pink filled square in the last panel denotes the median values of and , for the entire sample, including those sources with solid Ne line detections but no [S iv], and therefore it is not located at the center of the points in the plot (which only include sources with solid [S iv] detections).