Wavelength dependence of the ratio R₂₁ of the equivalent widths of narrow DIBs (FWHM < 1.1Å, Table 2) in the lines of sight of HD 204827 and HD 183143 (Hobbs et al. 2008, 2009) confirmed in Fan et al. (2019). These DIBs are shown by red dots, except for the DIBs identified in Table 1 as ζ (Ensor et al. 2017; grey diamonds) or C₂ (Thorburn et al. 2003; black triangles). The straight full green line displays the linear fit of Eq. (1), and the dashed lines show the ±1σ limits.