Radial distribution of the slit-spectra-based oxygen abundances in NGC 628. Panel a: squares denote the abundances estimated through the R calibration for individual measurements, and circles are the S calibration-based abundances. The crosses mark the median value for (O/H)_R and (O/H)_S together in bins of 0.1 in R/R₂₅ and the scatter of each bin. Panel b: median value for the (O/H)_S (circles), (O/H)_R (squares), and (O/H)_R and (O/H)_S together (crosses). The median values for different abundances are obtained for the same bin, but the positions of symbols (circles and squares) are slightly shifted along the x-axis for clarity.