I’m working with anytree package in Python for creating and displaying a K-dimension tree, however the tree is not rendered properly.

This is part of my source code for my K-dimensional node:

import numpy as np
from anytree import NodeMixin


class KdData:
    def __init__(self) -> None:
        self.key = None


class KdNode(KdData, NodeMixin):
    def __init__(self, parent=None, children=None, axis: int = 0) -> None:
        super().__init__()
        self.axis = axis
        self.parent = parent
        if children:
            self.children = children

    def set_key(self, key):
        self.key = np.array(key)
        if self.key is not None:
            self.children = [
                KdNode(axis=(self.axis + 1) % self.key.shape[0]),
                KdNode(axis=(self.axis + 1) % self.key.shape[0])
            ]

    def get_left_node(self):
        if self.key is None:
            return None
        return self.children[0]

    def get_right_node(self):
        if self.key is None:
            return None
        return self.children[1]

    def insert(self, key):
        if self.key is None:
            self.set_key(key)
        elif key[self.axis] < self.key[self.axis]:
            self.get_left_node().insert(key)
        else:
            self.get_right_node().insert(key)

    # Other functions are omitted because it's not relavant to the question

Then this is the tree I created.

tree = KdNode()
tree.set_key(np.array([5,6]))
tree.insert([4,7])
tree.insert([17,16])
tree.insert([7,8])
tree.insert([1,4])
tree.insert([9,13])

And this is my exporter:

def node_attribute(node):
    att = ""
    att += "shape=plaintext"
    return att

def edge_att(source, target):
    att = ""
    if target.key is None:
        att += "style=invis"
    return att

from anytree.exporter.dotexporter import DotExporter, UniqueDotExporter

dot_obj = UniqueDotExporter(tree,
    nodenamefunc=lambda node: node.key if node.key is not None else "",
    nodeattrfunc=lambda node: node_attribute(node),
    edgeattrfunc=lambda src, tgt: edge_att(src, tgt))

My obtained result is here. The expected result is that [4 7] node and [17 16] node are in the same rank.

Why was my K-dimensional tree not rendered properly?