code: def draw_line(cvs, p1, p2, antialias):
      """Draw a single line.
  
      Parameters
      ----------
      cvs: canvas
        A Datashader canvas
      p1: tuple
        The first vertex of the line
      p2: tuple
        The second vertex of the line
      antialias: boolean
        To anti-alias or not is the question
  
      Returns
      -------
      agg: A Datashader aggregator (xarray)
  
      """
      xs, ys = np.array([p1[0], p2[0]]), np.array([p1[1], p2[1]])
      points = pd.DataFrame({"x": xs, "y": ys, "val": 5.0})
      return cvs.line(
          points,
          "x",
          "y",
          agg=ds.reductions.max("val"),
          antialias=antialias,
      )
  
imports: import os
  import xarray
  import datashader
  import pandas
  import numpy
  import pytest
label_desc: Use image node_id 1 for calling a global function with example usage: draw_line(cvs, p1, p2, antialias) and returns: cvs