Contour plot of the 2D PDFs of C¹⁸O vs. ¹³CO (left) and C¹⁸O vs. HCO⁺ (right). The PDFs of the total dataset are shown as black dotted contours. The contributions of the 7 groups resulting from the grouping discussed in the text are shown in contours coloured according to Fig. 11 (right). In addition, the PDF maximum of each group is shown as a cross with the same colour as the group.