Comparison of the CRB reference precisions for different combinations of two species and all their available lines. The layout of this figure is identical to Fig. 11, except for the combination of studied lines: the first column shows the precision when the (1 − 0) and (2 − 1) lines of the ¹³CO and C¹⁸O isotopologues are used. The last two columns study a combination of the one CO isotopologue with one HCO⁺ isotopologue: ¹³CO and H¹²CO⁺ on the second column and C¹⁸O and H¹³CO⁺ on the third column.