Dependence of the log(U) versus 12 + log(O/H) relation on the stellar mass surface density (left) and Hα surface brightness (right). The ionization parameters and metallicities shown were derived using all five line ratios. The contours represent five density levels equally spaced in the logarithmic space from the 16th percentile to the 84th percentile. The dotted-dashed lines indicate the median relations in different bins.