Left: [C III] λ1907/He II – [O III] λ1666/He II diagnosis. The Gutkin et al. (2016) models plotted correspond to a pure photoionisation scenario due to massive stars with log(U_s) =  − 1, ξ_d = 0.1, m_up = 300 M_⊙, and $0.14<(\mathrm{C}/\mathrm{O})/{(\mathrm{C}/\mathrm{O})}_{\odot }<0.27$. The coloured area represents the region where the values of (C/O)/(C/O)_⊙ span vertically between 0.14 and 0.27 (both values marked by black lines), while the gas-phase metallicity varies horizontally between 12 + log(O/H) = 7 and 8.75. Right: C IV λλ1548, 51/[C III] λ1907 – C IV λλ1548, 51/He II diagnosis for those galaxies where C IV λλ1548, 51 is detected. The rainbow lines correspond to photoionisation models with gas density n_H = 10² cm⁻³, tracing a metallicity gradient from 12 + log(O/H) = 7 to 8.75. Three different paths are shown, each representing a different ionisation parameter log(U_s), ranging from −3 to −1. All models assume a C/O abundance ratio of 0.1(C/O)_⊙. The observed scatter reflects a gradient in the ionisation parameter as C IV increases linearly relative to the other lines in a pure photoionisation scenario. In addition, the two AGNs (ID:1051 and ID:1056) present a significantly higher C IV/[C III] ratio, distinguishing them from the rest of the sample. All the points in the two diagnoses lie in the region of very low metallicity (12 + log(O/H) < 7.7), which are similar to the measured values.