求助用jfreechart画时序图问题！大神们帮帮忙！~

用jfreechart画的时序图，为什么起始位置是在最右边而不是在坐标轴的开始位置？怎么修改呢？
 private static JFreeChart createChart(String chartContent,String title,String yaxisName){  
        //创建时序图对象  
        timeSeries1 = new TimeSeries(chartContent,Millisecond.class); //添加一条走势曲线
        timeSeries2 = new TimeSeries(chartContent,Millisecond.class);
        TimeSeriesCollection timeseriescollection = new TimeSeriesCollection();  
        timeseriescollection.addSeries(timeSeries1);
        timeseriescollection.addSeries(timeSeries2);
        JFreeChart jfreechart = ChartFactory.createTimeSeriesChart(title,"Time",yaxisName,timeseriescollection,true,true,false);  
        XYPlot xyplot = jfreechart.getXYPlot();  //将timeseriescollection中的坐标给到xyplot中
        xyplot.setBackgroundPaint(Color.BLACK);
        //纵坐标设定  
        ValueAxis valueaxis = xyplot.getDomainAxis();  
        //自动设置数据轴数据范围  
        valueaxis.setAutoRange(true);
        
        //数据轴固定数据范围 30s  
        valueaxis.setFixedAutoRange(30000D);
       // valueaxis.setLowerBound(10000);
        //valueaxis.setUpperBound(90000);
        //xyplot.setAxisOffset(new RectangleInsets(50.0, 0, 0, 50.0));
  
        valueaxis = xyplot.getRangeAxis();  
        valueaxis.setRange(70D,100D);  
        valueaxis.setPositiveArrowVisible(true); 
  
        return jfreechart;  
      }  
  
    public void run()  
    {  
        while(true)  
        {  
        try  
        {  
            timeSeries1.add(new Millisecond(), randomNum());  
            timeSeries2.add(new Millisecond(), randomNum()); 
            Thread.sleep(300);  
        }  
        catch (InterruptedException e)  {   }  
        }         
    }  
      
    private long randomNum()  
    {     
        //System.out.println((Math.random()*20+80));        
        return (long)(Math.random()*20+80);  
    }   jfreechart 时序图 --------------------编程问答-------------------- --------------------编程问答--------------------

补充：Java ,  Java相关