How to draw a hierarchical diagram that contains groups?

Question

I'm currently attempting to use JGraphX to create a diagram in a Swing application. My goal is to achieve something similar to this (this should be a directed graph, I just didn't have arrowheads in my mockup tool):

Using JGraphX, the closest I can achieve is this:

The "Bottom" node should be below the grouping, but it renders next to it.

I'm hoping that somebody can either find the problem with my code, or recommend a different diagram / graphing Java library that can achieve the output I'm looking for.

Here is my current drawing code:

import com.mxgraph.layout.hierarchical.mxHierarchicalLayout;
import com.mxgraph.swing.mxGraphComponent;
import com.mxgraph.view.mxGraph;

import javax.swing.*;
import java.awt.*;

public class StaticGraphTest extends JFrame {
    public StaticGraphTest() {
        mxGraph graph = new mxGraph();
        Object parent = graph.getDefaultParent();

        graph.getModel().beginUpdate();
        try {
            Object item1 = graph.insertVertex( parent, null, "Top", 0, 0, 80, 30 );
            Object grouping = graph.insertVertex( parent, null, "", 0, 0, 80, 100 );
            graph.insertEdge( parent, null, "", item1, grouping );

            //Add items inside grouping
            Object sub1 = graph.insertVertex( grouping, null, "Subitem A", 0, 0, 80, 30 );
            graph.insertEdge( grouping, null, "", grouping, sub1 );
            Object sub2 = graph.insertVertex( grouping, null, "Subitem B", 0, 0, 80, 30 );
            graph.insertEdge( grouping, null, "", grouping, sub2 );
            
            Object item2 = graph.insertVertex( parent, null, "Bottom", 0, 0, 80, 30 );
            graph.insertEdge( parent, null, "", grouping, item2 );

            mxHierarchicalLayout layout = new mxHierarchicalLayout( graph );
            //layout.setResizeParent( true );
            layout.setOrientation( SwingConstants.NORTH );
            layout.execute( parent );
        } finally {
            graph.getModel().endUpdate();
        }

        mxGraphComponent graphComponent = new mxGraphComponent( graph );

        //graphComponent.setEnabled( false ); //Make diagram read-only
        getContentPane().setLayout( new BorderLayout() );
        getContentPane().add( graphComponent, BorderLayout.CENTER );
    }

    public static void main( String[] args ) {
        EventQueue.invokeLater( ()->{
            StaticGraphTest frame = new StaticGraphTest();
            frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
            frame.setSize( 400, 320 );
            frame.setVisible( true );
        } );
    }
}