Contour plot of the 2D PDFs of ¹³CO vs. ¹²CO (left), and ¹³CO vs. C¹⁸O (right). The PDFs of the total dataset are shown as black dotted contours, while the PDFs of the individual clusters are shown as solid contours coloured according to the colour coding of clusters in Fig. 2. On the right panel clusters CO-1 to CO-5 have been grouped (grey contours) for better readability. The thin vertical and horizontal dashed lines show the median 4σ noise levels, while the coloured crosses show the positions of the PDF maxima for each cluster.