T_e(Cl²⁺) as a function of T_e(N⁺) (top), T_e(S²⁺) (middle), and T_e(O²⁺) (bottom) obtained from the photoionisation models described in the text. Each point represents an individual model colour-coded by its mean ionisation degree, parameterised by the O²⁺/O ratio. The red line is a linear fit to the points. The small panels represent the values of σ to the fit in 1000 K bins. The mean σ is shown inside the panels.