code: def tree_graph() -> StellarGraph:
      nodes = pd.DataFrame(
          index=["root", "0", 1, 2, "c1.1", "c2.1", "c2.2"]
      )
      edges = pd.DataFrame(
          [
              ("root", 2),
              ("root", 1),
              ("root", "0"),
              (2, "c2.1"),
              (2, "c2.2"),
              (1, "c1.1"),
          ],
          columns=["source", "target"],
      )
  
      return StellarDiGraph(nodes, edges)
  
imports: from stellargraph import StellarGraph, StellarDiGraph, IndexedArray
  import networkx
  import pandas
  import numpy
  import random
  import pytest
label_desc: Use image node_id 15 for calling a global function with example usage: tree_graph() and returns: StellarDiGraph