code: def test_sankey_label_index(self):
      sankey = Sankey(
          (
              [
                  (0, 2, 5),
                  (0, 3, 7),
                  (0, 4, 6),
                  (1, 2, 2),
                  (1, 3, 9),
                  (1, 4, 4),
              ],
              Dataset(enumerate("ABXYZ"), "index", "label"),
          )
      ).opts(label_index="label")
      plot = mpl_renderer.get_plot(sankey)
      labels = plot.handles["labels"]
  
      text_data = {
          "x": np.array([18.75, 18.75, 1003.75, 1003.75, 1003.75]),
          "y": np.array(
              [125.454545, 375.454545, 48.787879, 229.090909, 430.30303]
          ),
          "text": ["A - 18", "B - 15", "X - 7", "Y - 16", "Z - 10"],
      }
      for i, text in enumerate(labels):
          self.assertEqual(text.xy[0], text_data["x"][i])
          self.assertEqual(text.xy[1], text_data["y"][i])
          self.assertEqual(text.get_text(), text_data["text"][i])
  
file_path: holoviews/holoviews/tests/plotting/matplotlib/test_sankey.py
imports: import numpy
  from holoviews.core.data import Dataset
  from holoviews.element import Sankey
  from .test_plot import TestMPLPlot, mpl_renderer
label_desc: Use image node_id 2 for calling the TestSankeyPlot obj's underlying member method code with example usage: obj.test_sankey_label_index() without return types