如何调整子combo的位置，使其居中于父combo?

[ltdys]

现在的效果：

期待的效果：

this.chartData = {
nodes: [
{ id: 'node10', label: 'Node 10', comboId: 'combo1' },
{ id: 'node11', label: 'Node 11', comboId: 'combo1' },
{ id: 'node12', label: 'Node 12', comboId: 'combo1' },
{ id: 'node13', label: 'Node 13', comboId: 'combo1' },
{ id: 'node14', label: 'Node 14', comboId: 'combo1' },
{ id: 'node15', label: 'Node 15', comboId: 'combo1' },
{ id: 'node16', label: 'Node 16', comboId: 'combo1' },
{ id: 'node21', label: 'Node 21', comboId: 'combo2' },
{ id: 'node22', label: 'Node 22', comboId: 'combo2' },
{ id: 'node23', label: 'Node 23', comboId: 'combo2' },
{ id: 'node24', label: 'Node 24', comboId: 'combo2' },
{ id: 'node25', label: 'Node 25', comboId: 'combo2' },
{ id: 'node26', label: 'Node 26', comboId: 'combo2' },
{ id: 'node30', label: 'Node 30', comboId: 'combo3' },
{ id: 'node31', label: 'Node 31', comboId: 'combo3' },
{ id: 'node32', label: 'Node 32', comboId: 'combo3' },
{ id: 'node33', label: 'Node 33', comboId: 'combo3' },
{ id: 'node34', label: 'Node 34', comboId: 'combo3' },
],
edges: [
],
combos: [
{ id: 'combo1', label: 'Level 0' },
{ id: 'combo2', label: 'Level 1', parentId: 'combo1' },
{ id: 'combo3', label: 'Level 2', parentId: 'combo2' },
],
};
// 创建图实例
const graph = new G6.Graph({
container: 'container',
width: 800,
height: 600,
modes: {
default: ['drag-canvas', 'zoom-canvas', 'drag-combo'],
},
defaultNode: {
size: 30,
style: {
fill: '#DEE9FF',
stroke: '#5B8FF9',
},
},
defaultEdge: {
style: {
stroke: '#e2e2e2',
},
},
defaultCombo: {
type: 'rect',
style: {
fill: '#f0f0f0',
stroke: '#ccc',
},
labelCfg: {
position: 'top',
},
},
comboStateStyles: {
active: {
stroke: '#000',
},
},
layout: {
type: 'comboCombined',
comboPadding: 20,
outerLayout: new G6.Layout['grid']({
rows: 1,
cols: 3,
}),
innerLayout: new G6.Layout['grid']({
rows: 4,
cols: 4,
}),
},
});

  // 加载数据
  graph.data(this.chartData);
  // 渲染图
  graph.render();

子combo默认是在左上位置的，想调整成居中显示应该怎么设置？