Fitting examples in one of the 3D bins. The dashed red lines are the best-fit lines given by the maximum-likelihood method. The orange shaded regions indicate the 1σ uncertainties of the linear models. The dash-dotted blue lines are obtained by fixing the slopes using the values from an F99 extinction curve with R_V = 3.1 during the fit. The error bars indicate the measurement uncertainties in different logarithmic line ratios. Left: reddening relation between Hα/Hβ and Hα/Hγ. Middle: reddening relation between Hα/Hβ and [S III]/[O III]. Right: reddening relation between Hα/Hβ and [N II]/[O II].