Median Σ_SFR for SDSS galaxies as a function of position in the [S II] BPT diagram. There is a strong trend for galaxies with higher values of Σ_SFR to be shifted toward lower values of [S II]λλ6717, 3731/Hα at fixed [O III]λ 5007/Hβ (Masters et al. 2016). In addition, we indicate the running median for local H II regions (Pilyugin & Grebel 2016) with the dotted pink line. The running median line ratios are calculated in bins of H II-region electron temperature. The hiionly model from Sanders et al. (2017) is shown with the dashed black line, representing the ensemble average emission from H II regions in star-forming galaxies in the absence of DIG emission. Finally, the running median for “DIG”-like (i.e., low Hα surface brightness) spaxels from the SDSS/MaNGA DIG galaxy sample used in Sanders et al. (2017) is shown as the solid dark-gray curve. The running median line ratios for the “DIG”-like curve are calculated in bins of ([O III]λ 5007/Hβ)/([N II]λ6584/Hα), which correlates with nebular metallicity in local H II regions (Pettini & Pagel 2004).