code: def test_simple_heatmap(self):
      msno.heatmap(self.simple_df)
      return plt.gcf()
  
imports: import unittest
  import pandas
  import numpy
  import pytest
  import sys
  import missingno
  import matplotlib.pyplot
label_desc: Use image node_id 2 for calling the TestHeatmap obj's underlying member method code with example usage: obj.test_simple_heatmap() and returns: plt