时间型条型图，怎么修改提示框中显示的内容？echarts 折线配置项内容和展示

时间型条型图，数据全部在series中的data渲染，当某些数据项为null时，并不能很好的显示，怎么修改提示框中的显示的内容为‘’或者 ‘-’

      option = {
    tooltip: {
        trigger: 'axis',
        axisPointer: {
            label: {
                show: true,
                precision: '2',
                formatter: function(params) {
                    let str = new Date(params.value).toJSON()
                    return str.slice(0, 10) + ' ' + str.slice(11, 19)
                }
            }
        },
        backgroundColor: 'rgba(255, 255, 255, .8)',
        borderColor: '#ddd',
        textStyle: {
            color: '#333'
        },
        extraCssText: 'box-shadow: 0 0 3px rgba(0, 0, 0, 0.3);'

    },
    grid: {
        left: 70,
        right: 30,
        top: 20
    },
    xAxis: {
        type: 'time',
        interval: 1000 * 60 * 2,
        splitLine: {
            show: false
        },
        axisLabel: {
            formatter: '{value}'
        },
        axisTick: {
            length: 2
        }
    },
    yAxis: {
        type: 'value',
        axisLabel: {
            formatter: `{value} %`
        }
    },
    series: [{
        name: 'demo',
        type: 'line',
        connectNulls: false,
        smooth: true,
        symbol: 'circle',
        symbolSize: 5,
        showSymbol: false,
        lineStyle: {
            normal: {
                width: 1
            }
        },
        areaStyle: {
            normal: {
                color: new echarts.graphic.LinearGradient(0, 0, 0, 1, [{
                    offset: 0,
                    color: 'rgba(0, 136, 212, 0.3)'
                }, {
                    offset: 0.8,
                    color: 'rgba(0, 136, 212, 0)'
                }], false),
                shadowColor: 'rgba(0, 0, 0, 0.1)',
                shadowBlur: 10
            }
        },
        itemStyle: {
            normal: {
                color: 'rgb(0,136,212)',
                borderColor: 'rgba(0,136,212,0.2)',
                borderWidth: 12

            }
        },
        data: [
            [1513578540000, 0.6],
            [1513578600000, 0.3],
            [1513578660000, 0.3],
            [1513578720000, 0.3],
            [1513578780000, 0.2],
            [1513578840000, 0.4],
            [1513578900000, 0.2],
            [1513578960000, 0.4],
            [1513579020000, 0.2],
            [1513579080000, 0.4],
            [1513579140000, 0.4],
            [1513579200000, 0.3],
            [1513579260000, 0.3],
            [1513579320000, 0.2],
            [1513579380000, 0.5],
            [1513579440000, 0.1],
            [1513579500000, 0.3],
            [1513579560000, 0.4],
            [1513579620000, 0.3],
            [1513579680000, 0.2],
            [1513579740000, 0.4],
            [1513579800000, 0.2],
            [1513579860000, 0.2],
            [1513579920000, 0.3],
            [1513579980000, 0.4],
            [1513580040000, null],
            [1513580100000, null],
            [1513580160000, 0.4],
            [1513580220000, 0.4],
            [1513580280000, 0.2],
            [1513580340000, 0.3],
            [1513580400000, 0.1],
            [1513580460000, 0.3],
            [1513580520000, 0.4],
            [1513580580000, 0.3],
            [1513580640000, 0.4],
            [1513580700000, 0.4],
            [1513580760000, 0.2],
            [1513580820000, 0.1],
            [1513580880000, 0.3],
            [1513580940000, 0.4],
            [1513581000000, 0.7],
            [1513581060000, 0.6],
            [1513581120000, 0.6],
            [1513581180000, 0.5],
            [1513581240000, 0.7],
            [1513581300000, 0.6],
            [1513581360000, 0.5],
            [1513581420000, 0.5],
            [1513581480000, 0.6],
            [1513581540000, 0.4],
            [1513581600000, 0.5],
            [1513581660000, 0.5],
            [1513581720000, 0.6],
            [1513581780000, 0.6],
            [1513581840000, 0.6],
            [1513581900000, 0.6],
            [1513581960000, 0.5],
            [1513582020000, 0.6],
            [1513582080000, 0.5],
            [1513582140000, 0]
        ]
    }]
};