Line ratio diagram, showing ([Ne III]+[O II])/Hδ versus [Ne III]/[O II], featuring GN-z11. The yellow diamond denotes the line ratios derived from the medium-dispersion G395M grating, and the cyan diamond uses the low-dispersion prism spectrum (where we have corrected for the blending of [Ne III] λ3869 with He Iλ3889 using the flux ratio from the grating). The background grey 2D probability density function shows the subset of SDSS galaxies with 0.03 < z < 0.1 for which [Ne III] λ3869, [O II] λλ3726, 3729, and Hδ are all detected with S/N > 5. Purple squares show z > 5.5 galaxies from Cameron et al. (2023) after adjusting the reported ratios to be in terms of Hδ by assuming a fixed value of Hδ/Hβ = 0.268. Solid lines show model grids from Gutkin et al. (2016), plotted for nine different values of metallicity (Z/Z_⊙ = 0.04, 0.07, 0.15, 0.30, 0.45, 0.60, 0.75, 1.0, 1.5), as indicated by the different colours, and seven values of the ionisation parameter, indicated by marker sizes, in steps of 0.5 from log₁₀U = −4.0 (smallest) to log₁₀U = −1.0 (largest).