Correlation diagram for C₂ DIBs with the same symbols and definitions as in Figs. 1–2, where the horizontal axis displays the normalised correlation factor, kn₆₀₁₀ (equivalent to kn₆₀₀₉, Appendix B.3), and the vertical axis displays the ratio of the DIB equivalent widths in the line of sight HD 204827 to HD 183143 (Eq. (1)). Reference ζσ and σ DIBs of Fan et al. (2022) have been added to show the whole range of variation of kn₆₀₁₀ and R₂₁. The selection criteria (Eqs. (5,6); dotted blue lines) distinguish C₂ DIBs well from ζ DIBs. The intermediate position of C₂-related DIBs (Table B.4, green diamonds) between C₂ and ζ DIBs, is clearly visible, as is the peculiar position of two reference DIBs, λλ4726, 5170, and of λ5849 at the boundary of ζ DIBs in Fig. 2 of Fan et al. (2022).