Relation of the HCO⁺ (top row), H¹³CO⁺ (middle row), and HC¹⁸O⁺ (bottom row) molecular abundances to cloud physical parameters: kinetic temperature (left column), extinction (middle column), and molecular hydrogen density (right column). The dataset is color-coded according to the molecular cloud of the points, as indicated in the legends. The black lines show the linear correlations between molecular abundances and molecular hydrogen density for the corresponding whole samples, with the correlation parameters indicated in the legends.