C III] λ1909 rest-frame equivalent widths for low- and high-z star-forming galaxies with measured metallicities. The filled blue square shows J1154+2443 and the black filled triangles denote the low-redshift galaxies studied by Senchyna et al. (2017) and reanalysed by Chevallard et al. (2018). Red symbols show the z = 2−4 galaxies (stacked spectra) analysed by Nakajima et al. (2018a, b) and magenta symbols show the z = 1.844, 2.0, and 7.73 galaxies from Berg et al. (2018), Erb et al. (2010), and Stark et al. (2017), respectively. The smaller open triangles and filled diamonds represent the other low-z and z = 2−4 galaxies, respectively, compiled in Nakajima et al. (2018a). Green curves present photoionization models using single- (dotted) and binary- (long dashed) stellar populations with different ionization parameters (Nakajima et al. 2018a).