Abundance of O⁺ (top panel), O²⁺ (middle panel), and O²⁺/O⁺ (bottom panel) versus total abundance. All the abundances are presented in the form of 12+log(abundance). The uncertainties of abundance measurements are illustrated via the gray error bars, and in the bottom panel, the error bars are calculated via error propagation. The dashed line in the bottom panel demonstrates 12 + log(O²⁺/H) = 12 + log(O⁺/H). Data points are color-coded by T_e [S II].