How to set a fixed value for the echars Y-axis scale, for example, if you want to add a 3.9 between 0 and 10 but do not change the rest, for example, if you want to add a 3.9 between 0 and 10?

xAxis: {
            type: "category",
            data: data.map(function (item) {
                return item.date;
            }),
            axisLabel: {
                interval:5,
                formatter: function (value, idx) {
                    return value;
                }
            },
            splitLine: {
                show: true
            },
            boundaryGap: false
        },
        yAxis: {
            max:20,
            axisLabel: {
                formatter: function (val) {
                    return val
                }
            },
            axisPointer: {
                label: {
                    formatter: function (params) {
                        //1
                        return params.value.toFixed(1) + "mmol/L";
                    }
                }
            },
            //Y
            splitNumber: 2,
            splitLine: {
                show: true
            },
            show:true,
            name:"mmol/L"
        },
        series: [
        {
            smooth:true,
            type: "line",
            //
            data: data.map(function (item) {
                return 10;
            }),
            hoverAnimation: false,
            lineStyle:{
                normal:{
                    color: "-sharp000",
                    width:1
                }
            },
            showSymbol: false
        },{
            smooth:true,
            type: "line",
            //
            data: data.map(function (item) {
                return 3.9;
            }),
            hoverAnimation: false,
            lineStyle:{
                normal:{
                    color: "-sharp000",
                    width:1
                }
            },
            showSymbol: false
        },{
            //
            smooth:true,
            name: "L",
            type: "line",
            data: data.map(function (item) {
                return 0;
            }),
            lineStyle: {
                normal: {
                    opacity: 0
                }
            },
            stack: "confidence-band",
            symbol: "none",
        }, {
            //
            smooth:true,
            name: "U",
            type: "line",
            data: data.map(function (item) {
                console.log(item.value,"item.value")
                return item.value + 3;
            }),
            lineStyle: {
                normal: {
                    opacity: 0
                }
            },
            areaStyle: {
                normal: {
                    color: "-sharpccc"
                }
            },
            stack: "confidence-band",
            symbol: "none"
        }, {
            smooth:true,
            type: "line",
            //
            data: data.map(function (item) {
                return item.value;
            }),
            hoverAnimation: true,
            symbolSize: 6,
            itemStyle: {
                normal: {
                    color: "-sharp00f"
                }
            },
            showSymbol: false
        }]