怎么设置两条线的中间区域为阴影echarts 折线配置项内容和展示

已经画出两条线来，但是怎么才能设置中间区域为阴影

      option = {
    title: {
        text: '需求趋势图'
    },
    tooltip: {
        trigger: 'axis',
        axisPointer: {
            type: 'cross',
            animation: false,
            label: {
                backgroundColor: 'yellow',
                borderColor: 'red',
                borderWidth: 1,
                shadowBlur: 0,
                shadowOffsetX: 0,
                shadowOffsetY: 0,
                textStyle: {
                    color: 'red'
                }
            }
        }
    },
    legend: {
        data: ['观测值', '拟合值', '预测值', '置信区间']
    },
    toolbox: {
        feature: {
            saveAsImage: {}
        }
    },
    grid: {
        left: '3%',
        right: '4%',
        bottom: '3%',
        containLabel: true
    },
    xAxis: [{
        type: 'category',
        boundaryGap: false,
        data: date_time
    }],
    yAxis: [{
        type: 'value'
    }],
    series: [{
        name: '观测值',
        type: 'line',
        data: [-2499.0, -5504.0, -5057.4, -936.0, -8016.0, -12733.0, -1428.0, -2260.0, -19664.0, -11686.0],
        hoverAnimation: false,
        symbolSize: 6,
        itemStyle: {
            normal: {
                color: '#c23531'
            }
        },
        showSymbol: false
    }, {
        name: '拟合值',
        type: 'line',
        data: [-3519, -6245, -6078, -1956, -9036, -13753, -2448, -3280, -20684],
        hoverAnimation: false,
        symbolSize: 6,
        itemStyle: {
            normal: {
                color: 'blue'
            }
        },
        showSymbol: false

    }, {
        name: '预测值',
        type: 'line',
        data: [-12706.77777778 - 13727.55555556 - 14748.33333333 - 15769.11111111 - 16789.88888889],
        hoverAnimation: false,
        symbolSize: 6,
        itemStyle: {
            normal: {
                color: 'yellow'
            }
        },
        showSymbol: false
    }, {
        name: '最大趋势',
        type: 'line',
        data: [null, null, null, null, null, null, null, null, null, null, -11686, 3084.91863283, 8605.27568121, 12603.6871875, 15814.2817101, 18521.4177653],
        lineStyle: {
            normal: {
                opacity: 0
            }
        },
        areaStyle: {
            normal: {
                color: '#ccc'
            }
        },
        stack: 'confidence-band',
        symbol: 'none'
    }, {
        name: '最小趋势',
        type: 'line',
        data: [null, null, null, null, null, null, null, null, null, null, -11686, -28498.4741884, -36060.3867923, -42100.3538542, -47352.5039323, -52101.1955431],
        lineStyle: {
            normal: {
                opacity: 0
            }
        },
        areaStyle: {
            normal: {
                color: '#ccc'
            }
        },
        stack: 'confidence-band',
        symbol: 'none'
    }]
};