Logarithm of O32 versus the logarithm of R23. The black points are galaxies from our sample, open symbols reflect galaxies with S/N (Hβ) < (3), while the green stars are the confirmed LyC leakers from Izotov et al. (2016a); Izotov et al. (2016b); Izotov et al. (2018a). The squares that are connected by solid (log U  <  −2) and dashed coloured lines show the position in this diagram of the nebular model of Gutkin et al. (2016) for constant gas-phase metallicity (see legend) and with increasing log U towards the upper right, with −4  <  log U  <  −1 and step size 0.5.