Ratio of the EW of the 1318 nm NIR DIB divided by the EW of optical DIBs or E(B – V) plotted against (1) λλ5797/5780 ratio, (2) Ca I (4226 Å)/Ca II (3933 Å), (3) CH (4300 Å)/CH+ (4232 Å) and (4) Na I (5895 Å)/Ca II (3933 Å). Blue circles are for the whole sample, and orange squares show the single component dominated DIBs. The dashed lines show upper and lower error bounds obtained by fitting with a second order polynomial.