 unconnected class nodes from the main Diagram
    diagram.model.removeNodeDataCollection(singlesArray);

    // Display the unconnected classes in a separate Diagram
    var singletons = new go.Diagram('mySingletons', {
      nodeTemplate: diagram.nodeTemplate, // share the node template with the main Diagram
      layout: new go.GridLayout({
        wrappingColumn: 1, // put the unconnected nodes in a column
        spacing: new go.Size(3, 3)
      }),
      model: new go.Model(singlesArray) // use a sepa