Predicted auroral [OII] and trans-auroral [SII] line ratios for the 896 models considered in this work. The hydrogen number density ranges from 10² cm⁻³ to 10⁵ cm⁻³, and the reddening ranges from E(B − V) = 0 mag to E(B − V) = 1 mag, both of which are indicated with arrows in the diagram. The colour of the markers represents the ionisation parameter, where log U = −3.8 is marked with purple and log U = −2 is marked with yellow. The size of the markers represents the gas metallicity, where the smallest markers correspond to 0.5 Z_⊙ and the largest correspond to 2 Z_⊙. The shapes of the markers represent different shapes of the ionising SED, where squares, stars, circles, and triangles correspond to SEDs with a mean energy of ionising photon of 2.56, 2.65, 3.15, and 4.17 Ryd, respectively. See text for additional details about the models.