Left: 1D PDF of the HCO+ (1–0) line intensity, comparing the full dataset PDF (thick dashed black line), the contribution of each of the groups defined in Sect. 5.2 (coloured thick lines), and the contribution of each individual cluster (thin coloured lines). The cluster contributions are coloured according to the group to which they belong. For readability, we have separated groups HCO⁺-1 to HCO⁺-4 and their constitutive clusters (left panels) and groups HCO⁺-5 to HCO⁺-7 and their constitutive clusters (right panels). Right: map of the 7 groups resulting from the grouping of consecutive clusters described in the text (Sect. 5.2).