添加dataZoom配置项后，connectNulls就失效了echarts 折线配置项内容和展示

connectNulls的默认值是false，即数据中包含空值的时候，不会用线把两端连起来，但是如果配置上了dataZoom:{}后，connectNulls就会变成true

      option = {
    title: {
        text: 'Step Line'
    },
    tooltip: {
        trigger: 'axis'
    },
    legend: {
        data:['Step Start', 'Step Middle', 'Step End']
    },
    grid: {
        left: '3%',
        right: '4%',
        bottom: '3%',
        containLabel: true
    },
    toolbox: {
        feature: {
            saveAsImage: {},
            dataZoom: { } //当增加了这一句后,data中的null就被连起来了
        }
    },
    xAxis: {
        type: 'category',
        data: ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
    },
    yAxis: {
        type: 'value'
    },
    series: [
        {
            name:'Step Start',
            type:'line',
            step: 'start',
            data:[120, 132, null, null, 90, 230, 210]
        },
        {
            name:'Step Middle',
            type:'line',
            step: 'middle',
            data:[220, 282, 201, 234, null, 430, 410]
        },
        {
            name:'Step End',
            type:'line',
            step: 'end',
            data:[450, 432, 401, 454, 590, null, 510]
        }
    ]
};