code: def threshold(requestContext, value, label=None, color=None):
      
      [series] = constantLine(requestContext, value)
      if label:
          series.name = label
      if color:
          series.color = color
      return [series]
docs: Takes a float F, followed by a label (in double quotes) and a color.
      (See ``bgcolor`` in the render\_api_ for valid color names & formats.)
  
      Draws a horizontal line at value F across the graph.
  
      Example::
  
          &target=threshold(123.456, "omgwtfbbq", "red")