Weighted linear fits for deriving N(OH⁺) for all targets except HD 80558 (Fig. 4). A couple of outliers can be noticed for some of the sightlines as in the cases for HD 185418 and HD 186745 which could well be caused by poor S/N (see Fig. 3) and/or possible contamination with some weak absorption features. This behavior is also seen in the measurements for the weaker OH⁺ lines in HD 79186. Setting the intercept to zero helps constrain the effect of these biases.