English: An example of the so-called "metro map" for the classical Iris flower data set ^[1]. The data set is approximated by the closest tree with some penalty for the excessive number of nodes, bending and stretching. The data points are projected into the closest node. For each node, the pie diagram of the projected points is prepared. The area of the pie is proportional to the number of the projected points. It is clear that the absolute majority of the samples of the different Iris species belong to the different nodes. Only a small fraction of Iris-virginica is mixed with Iris-versicolor (the mixed blue-grin nodes in the Fig.). Therefore, the three species of Iris (Iris setosa, Iris virginica and Iris versicolor) are separable by the unsupervising procedures of nonlinear principal component analysis. To discriminate them, it is sufficient just to select the corresponding nodes on the principal tree.