Residuals of observed N/O ratios with respect to the local best-fit relation, binned as a function of metallicity for the local (grey) and high-redshift (teal) samples. Error bars represent the uncertainties on the median values, while the shaded regions indicate the dispersion within each bin. The single data points represent the residuals of previously published literature sources with N/O and O/H measurements from rest-frame optical lines: five objects from Sanders et al. (2023) (CEERS program, cyan triangles); two galaxies from the EXCELS programme in Arellano-Córdova et al. (2025b) (red crosses); the Sunburst Arc from Welch et al. (2025) (orange square); the SGAS 1732 from Welch et al. (2024) (green plus); the GTO_2758_60001 from Zhang et al. (2026) (inverted dark red triangle); and the CECILIA D40 from Rogers et al. (2024) (purple diamond).