请问:  任意品种、周期加载1000个bar，然后在 第898 899 900的位置，用 plot.line 画三次线段，在901的位置如何用 plot.clear 删除掉这三个线段？

谢谢！

代码如下：这样写只能删掉898的位置画的线段，后面899 900两次还是会保留。

Params
    Numeric FastLength(5);// 短期指数平均线参数
    Numeric SlowLength(20);// 长期指数平均线参数
Vars
    Series<Numeric> AvgValue1; 
    Series<Numeric> AvgValue2;    
    Plot plt;
    Array<Numeric> x1Data;//x轴数据集
    Array<Numeric> y1Data;//y轴数据集
Events
    OnBar(ArrayRef<Integer> indexs)
    {
        AvgValue1 = AverageFC(Close,FastLength);
        AvgValue2 = AverageFC(Close,SlowLength);
        //plt.line(\"MA1\",AvgValue1);
        //plt.line(\"MA2\",AvgValue2); 
        Commentary(text(CurrentBar)); 
        if(currentbar ==900)
        {
        	Numeric i;
			For i=0 To 20
			{
				x1Data[i]= Date[i]+Time[i];
				y1Data[i]= AvgValue2[i]*1.01;
			}
			plt.line(\"vex0\",x1Data,y1Data);
        }    
        if(currentbar ==898)
        {
			Numeric i;
			For i=0 To 20
			{
				x1Data[i]= Date[i]+Time[i];
				y1Data[i]= AvgValue2[i]*1.007;
			}
			plt.line(\"vex\",x1Data,y1Data);

        }
        if(currentbar ==899)
        {
			Numeric i;
			For i=0 To 20
			{
				x1Data[i]= Date[i]+Time[i];
				y1Data[i]= AvgValue2[i]*1.007;
			}
			plt.line(\"vex\",x1Data,y1Data);

        }
        if(currentbar ==900)
        {
			Numeric i;
			For i=0 To 20
			{
				x1Data[i]= Date[i]+Time[i];
				y1Data[i]= AvgValue2[i]*1.007;
			}
			plt.line(\"vex\",x1Data,y1Data);

        }
        if(currentbar == 901)
        {
        	plt.clear(\"vex\");
        }
    }