public static void AddZAxis(Model3DGroup group,
                                    double length = 4, double thickness = 0.1)
        {
            MeshGeometry3D mesh   = new MeshGeometry3D();
            Point3D        origin = D3.Origin -
                                    D3.XVector(thickness / 2) -
                                    D3.YVector(thickness / 2) -
                                    D3.ZVector(thickness / 2);

            mesh.AddBox(origin,
                        D3.XVector(thickness), D3.YVector(thickness), D3.ZVector(length));
            group.Children.Add(mesh.MakeModel(Brushes.Blue));
        }