def to_networkx(self):
        """Return a NetworkX DiGraph object representing the single linkage tree.

        Edge weights in the graph are the distance values at which child nodes
        merge to form the parent cluster.

        Nodes have a `size` attribute attached giving the number of points
        that are in the cluster.
        """
        try:
            from networkx import DiGraph, set_node_attributes
        except ImportError:
            raise ImportError('You must have networkx installed to export networkx graphs')

        max_node = 2 * self._linkage.shape[0]
        num_points = max_node - (self._linkage.shape[0] - 1)

        result = DiGraph()
        for parent, row in enumerate(self._linkage, num_points):
            result.add_edge(parent, row[0], weight=row[2])
            result.add_edge(parent, row[1], weight=row[2])

        size_dict = {parent: row[3] for parent, row in enumerate(self._linkage, num_points)}
        set_node_attributes(result, size_dict, 'size')

        return result
Return a NetworkX DiGraph object representing the single linkage tree.

        Edge weights in the graph are the distance values at which child nodes
        merge to form the parent cluster.

        Nodes have a `size` attribute attached giving the number of points
        that are in the cluster.