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.

def add_line(self, start, end, color=(0.5, 0.5, 0.5), width=1):
        """
        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.
        """
        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)