如何让节点间的连线变为直角线echarts graph配置项内容和展示

如何让节点间的连线变为直角线

      option = {
    title: {
        text: 'Graph 简单示例'
    },
    tooltip: {},
    animationDurationUpdate: 1500,
    animationEasingUpdate: 'quinticInOut',
    series : [
        {
            type: 'graph',
            layout: 'none',
            symbolSize: 50,
            roam: true,
            label: {
                normal: {
                    show: true
                }
            },
            edgeSymbol: ['circle', 'arrow'],
            edgeSymbolSize: [4, 10],
            edgeLabel: {
                normal: {
                    textStyle: {
                        fontSize: 20
                    }
                }
            },
            data: [{
                name: '节点1',
                x: 300,
                y: 300
            }, {
                name: '节点2',
                x: 800,
                y: 300
            }, {
                name: '节点3',
                x: 550,
                y: 100
            }, {
                name: '节点4',
                x: 550,
                y: 500
            }],
            // links: [],
             links: [
                 //{
            //     source: 0,
            //     target: 1,
            //     symbolSize: [5, 20],
            //     label: {
            //         normal: {
            //             show: false
            //         }
            //     },

            // }, 
            {
                source: '节点1',
                target: '节点3'
            }, {
                source: '节点2',
                target: '节点3'
            }, {
                source: '节点2',
                target: '节点4'
            }, {
                source: '节点1',
                target: '节点4'
            }],
            lineStyle: {
                normal: {
                    opacity: 0.9,
                    width: 2,
                    curveness: 0
                }
            }
        }
    ]
};