Relationship between the significant clusters according to the single linkage algorithm, depicted according to their Mahalanobis distance in the IoM space (Paper I). The different colours represent a tentative classification into different substructures, namely six large groups (named A to F) and a set of individual clusters (in grey; see text for details). The order of the clusters along the x-axis is such that it facilitates the interpretation and readability of the dendrogram, and does not reflect internal properties. In our analyses (e.g. tables, etc.), we broadly follow this same order.