Diagnostics diagram of C III]/He IIλ1640 versus C IV/C III] (CHe2-C43). We plot our sample as blue squares. The left and right plots show ionisation models from Feltre et al. (2016), Gutkin et al. (2016) and Nakajima & Maiolino (2022), respectively, as yellow and light blue points (see Sect. 2.4). The green diamonds show He IIλ1640 detection from the VANDELS survey (Saxena et al. 2020), and all C IV detections from Mascia et al. (2023) as grey colour with squares and circles representing SF and AGN, respectively. The green squares show local analogues of high-redshift galaxies (from CLASSY survey; Mingozzi et al. 2024). We also show the black dashed and dotted demarcation lines between AGN, star-forming galaxies and composite line ratios from Hirschmann et al. (2023). Overall, we select five AGN on this diagram and we highlight these with a red circle. The magenta and cyan squares show a stacked spectrum for AGN and star-forming galaxies (see Sect 4.1).