K-S relation plot for all sample galaxies. The top left panel plots all galaxies and only ¹²CO(1−0) emission line data; the top right panel all galaxies and only ¹²CO(2−1) emission line data; the bottom left panel all galaxies and only ¹²CO(3−2) emission line data; and the bottom right panel all galaxies and all the available ¹²CO emission line data, at the common spatial resolution of 200 pc. Different symbols and/or colors indicate galaxies whose Σ_H2 has been derived from a given ¹²CO transition, as shown in the labels. The black lines refer to the OLS fits derived for different cases: the short-dotted line is for all galaxies and ¹²CO(1−0) emission line data, the point-dotted line for all galaxies and ¹²CO(2−1), the long-dotted line for all galaxies and ¹²CO(3−2), and the black solid line is for all galaxies and all the available ¹²CO lines. In the bottom right panel the ¹²CO(2−1) OLS fit line is hidden by total OLS fit line.