Log O32 versus the [O III]λ4363/[O III]λ5007 ratio (lower x-axis) and the electron temperature in the [O III] regime (lower x-axis) calculated using the nebular.ionic routine in Pyneb (Luridiana et al. 2015), assuming n_e = 100 cm³. The coloured lines indicate the predictions from the Gutkin et al. (2016) models of different metallicity as indicated by the colours, with increasing log U towards the upper right, and log U  <  −2 (solid line) and log U  <  −2 (dashed line), and step size of log U = 0.5 between the squares. The colours of the circles are used as in Fig. 17.