class_docstr: PosTag (Part of Speech Tagging Visualizer) Tests
docstr: Assert no errors occur when the visualizer is run on stack mode
docstr_tok: ["Assert","no","errors","occur","when","the","visualizer","is","run","on","stack","mode"]
code: def test_stack_mode(self):
      """
      Assert no errors occur when the visualizer is run on stack mode
      """
      check_nltk_data()
  
      _, ax = plt.subplots()
      tagged_docs = list(get_tagged_docs(sonnets))
  
      visualizer = PosTagVisualizer(stack=True, ax=ax)
      visualizer.fit(tagged_docs, y=["a", "b", "c"])
      visualizer.ax.grid(False)
  
      self.assert_images_similar(ax=ax)
  
imports: import pytest
  import matplotlib.pyplot
  from tests.base import VisualTestCase
  from tests.base import IS_WINDOWS_OR_CONDA
  from yellowbrick.text.postag import *
  from yellowbrick.exceptions import YellowbrickValueError
label_desc: Use image node_id 10 for calling the TestPosTag obj's underlying member method code with example usage: obj.test_stack_mode() without return types