Emission-line flux ratios of [O ii]λ3727/[O iii]λ5007 versus [Ne iii]λ3869/[O iii]λ5007. The filled circle with an arrow denotes the data of SDSS J1707+6443 (where the 3σ upper limit is adopted for the [O ii] flux), and cross symbols denote the data of the SDSS low-z (z ~ 0.7) type 1 quasars. The arrow at the upper-left corner in the panel is the reddening vector for observed data in the case of A_V = 1.0 mag, adopting the extinction curve of Cardelli et al. (1989). The grids are predicted emission-line flux ratios from Cloudy model runs, adopting Z_NLR = 3   Z_⊙ and varying the hydrogen density and the ionization parameter.