visualMap 如何实现居中实现echarts 折线配置项内容和展示

visualMap 如何实现居中实现

      option = {
    title: {
        text: 'Awesome Chart'
    },
    xAxis: {
        data: ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat']
    },
    yAxis: {},
    visualMap: {
        orient: 'horizontal',
        top: 0,
        /*right: 10,*/
        pieces: [{
            lt: 35,
            color: 'green',
            label: '优(<35)'
        }, {
            gte: 35,
            lt: 75,
            color: '#f1ee25',
            label: '良(35-75)'
        }, {
            gte: 75,
            lt: 115,
            color: '#ff9933',
            label: '轻度污染(75-115)'
        }, {
            gte: 115,
            lt: 150,
            color: '#cc0033',
            label: '中度污染(115-150)'
        }, {
            gte: 150,
            lt: 250,
            color: '#660099',
            label: '重度污染(150-250)'
        }, {
            gte: 250,
            lt: 999,
            color: '#7e0023',
            label: '严重污染(>250)'
        }],
        outOfRange: {
            color: '#999'
        }
    },
    series: [{
        type: 'line',
        data: [220, 182, 191, 234, 290, 330, 310]
    }]
};