code: def add_line(self, start, end, color=(0.5, 0.5, 0.5), width=1):
          
          source = vtk.vtkLineSource()
          source.SetPoint1(start)
          source.SetPoint2(end)
  
          vertexIDs = vtk.vtkStringArray()
          vertexIDs.SetNumberOfComponents(1)
          vertexIDs.SetName("VertexIDs")
          # Set the vertex labels
          vertexIDs.InsertNextValue("a")
          vertexIDs.InsertNextValue("b")
          source.GetOutput().GetPointData().AddArray(vertexIDs)
  
          mapper = vtk.vtkPolyDataMapper()
          mapper.SetInputConnection(source.GetOutputPort())
          actor = vtk.vtkActor()
          actor.SetMapper(mapper)
          actor.GetProperty().SetColor(color)
          actor.GetProperty().SetLineWidth(width)
          self.ren.AddActor(actor)
docs: Adds a line.
  
          Args:
              start: Starting coordinates for line.
              end: Ending coordinates for line.
              color: Color for text as RGB. Defaults to grey.
              width: Width of line. Defaults to 1.