class_docstr: PosTag (Part of Speech Tagging Visualizer) Tests
docstr: Ensure an exception is raised if the specified tagset is unknown
docstr_tok: ["Ensure","an","exception","is","raised","if","the","specified","tagset","is","unknown"]
code: def test_unknown_tagset(self):
      """
      Ensure an exception is raised if the specified tagset is unknown
      """
      with pytest.raises(YellowbrickValueError):
          PosTagVisualizer(tagset="brill")
  
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 2 for calling the TestPosTag obj's underlying member method code with example usage: obj.test_unknown_tagset() without return types