Upper panels: Sulphur and nitrogen chemical abundances versus total system metallicity for the PHANGS–MUSE sample and the other analysed literature catalogues. Lower panels: Relative abundances of sulphur and nitrogen with respect to oxygen, as a function of the total system metallicity. Solar values from Asplund et al. (2009) are reported as teal points for comparison. We display linear trends calibrated either using only the PHANGS-MUSE data or the entire dataset, which also includes literature data. The other log(S/O) versus 12 + log(O/H) trends are from Izotov et al. (2006) and Dors et al. (2016), with the former calibrated over BCD galaxies and the latter over a compilation of literature emission-line intensities of H II regions and star-forming galaxies and while considering sulphur ICF from Thuan et al. (1995). The log(N/O) versus 12 + log(O/H) trend is taken from Nicholls et al. (2017) and it is calibrated over a compilation of literature stellar data.