Dendrogram (upper panel) and mean abundance ratios  ⟨ [X/H] ⟩  (middle panel) and  ⟨ [X/Fe] ⟩  (bottom panel) for the four stellar groups from the tree clustering analysis (Sect. 5). They are classified as metal-poor (▴), intermediate abundance (under solar  △  or over solar °), or metal-rich (•) stars, and the same symbolism is adopted all over the paper. The dashed red line on the dendrogram represents a dissimilarity number of 0.9 dex.