Radial gradients of line ratios as functions of R_gal. The panels show (a) [N II]/Hα, (b) [S II]/Hα, and (c) [S II]/[N II]. Gray plus signs denote H II regions, and black squares represent the median values of binned H II regions, with scatter shown as error bars. Black lines correspond to linear fits to the median values, using errors as weights. Blue triangles and connecting lines represent the DIG sample from S. Wen et al. (2025).