public TestContextualLayout(PocGraph graph)
        {
            InitializeComponent();

            Graph          = graph;
            SelectedVertex = graph.Vertices.FirstOrDefault();
            DataContext    = this;
        }
        private void SelectedVertexChange_Click(object sender, RoutedEventArgs e)
        {
            var btn = e.Source as Button;

            SelectedVertex = btn.Tag as PocVertex;
        }