code: def test_vectorfield_alpha_op(self):
      vectorfield = VectorField(
          [(0, 0, 0, 1, 0), (0, 1, 0, 1, 0.2), (0, 2, 0, 1, 0.7)],
          vdims=["A", "M", "alpha"],
      ).opts(alpha="alpha")
      plot = bokeh_renderer.get_plot(vectorfield)
      cds = plot.handles["cds"]
      glyph = plot.handles["glyph"]
      self.assertEqual(
          cds.data["alpha"],
          np.array([0, 0.2, 0.7, 0, 0.2, 0.7, 0, 0.2, 0.7]),
      )
      self.assertEqual(
          property_to_dict(glyph.line_alpha), {"field": "alpha"}
      )
  
file_path: holoviews/holoviews/tests/plotting/bokeh/test_vectorfieldplot.py
imports: import numpy
  from holoviews.element import VectorField
  from holoviews.plotting.bokeh.util import property_to_dict
  from ..utils import ParamLogStream
  from .test_plot import TestBokehPlot, bokeh_renderer
label_desc: Use image node_id 4 for calling the TestVectorFieldPlot obj's underlying member method code with example usage: obj.test_vectorfield_alpha_op() without return types