Auroral-to-strong line ratio of [S II] (left) and [O II] (right) versus normalized Balmer decrement (HαHβ2.86) in one randomly chosen metallicity-ionization parameter bin, both in logarithm space. Each data point represents the line ratio of a sub-bin that is constructed by spaxels with similar Balmer decrements. The error bars describe the uncertainties of line ratios. The green line is the F99 correction relation with R_v = 3.1, which is generally steeper than the data distribution. The red line is our fitting for these data points, which considers both the x and y axes uncertainties simultaneously.