The relation between the abundance of CO, HCO⁺, and ﹩{N}_{{{\rm{H}}}_{2}}﹩ (circles from our work, squares from Lucas & Liszt 1996; Liszt & Lucas 1998). The red and black points denote sightlines using the ﹩{N}_{{\rm{H}}}/E(B-V)﹩ conversion factor of Nguyen et al. (2018) and Bohlin et al. (1978), respectively. The empty red circles denote sightlines with H I opacity correction. The black dotted lines show the mean values of molecular abundance (points with upper limits are not included).