rce": "tree",
          "transform": [
            {
              "type": "formula",
              "expr": "treeAncestors('tree', datum.id, 'root')",
              "as": "treeAncestors"
            }
          ]
        },
        {
          "name": "tree-ancestors-flatt",
          "source": "tree-ancestors",
          "transform": [{ "type": "flatten", "fields": ["treeAncestors"] }]
        },
        {
          "name": "selected-ancestors",
          "source": "tree-ancestors",
          "transform": 