Fig. 12
N/O vs. O/H. Left panel: the color codes the hardness of the ionizing radiation Q0 / 1. Right panel: the color codes the value of 
. The blue line is the fit of the Q0 / 1 > 0.55 regions (gray/red/orange points); see text for the corresponding values. The red dashed line corresponds to the fit by Pilyugin et al. (2012), the cyan dashed line to Vila Costas & Edmunds (1993), and the green dotted line to the fit by Dopita et al. (2013).
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