How to add a second series in jfreechart

[Manfizy]

Hi people,
Am using jfreechart to plot values that i that i read from a jtable.With the first series, my program is working ok but when i try to add a second series i get an error.

Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: java.lang.Double cannot be cast to java.lang.String

Anyone with an idea why am getting this kind of an error?
Below is a copy of the method that am using

public void plotGraph() {
    
final XYSeries series1 = new XYSeries("Poisoning");
final XYSeries series2 = new XYSeries("Concentration");
int i,num,num2;
double axis;
double values;

//Poisoning Profile 
num = Xenon.getRowCount();        

for (i = 0; i < num; i++){
    String time;
    time = (String) Xenon.getValueAt(i, 0); 
    if (time == null)
    {
    break;
    } 
    axis = Double.parseDouble(time);
    String tmp = (String) Xenon.getValueAt(i, 1);
    values =Double.parseDouble(tmp);

   series1.add(axis,values);
}

//Concentration profile
num2 = Temp.getRowCount();
for (int k = 0; k<num2; k++) {
String time2 = (String)Temp.getValueAt(k, 0);
if (time2 == null){
    break;
}
double axis2 = Double.parseDouble(time2);
String concentration = (String)Temp.getValueAt(k,3);
double xen =Double.parseDouble(concentration);

   series2.add(axis2, xen);
  // series2.add(15, 0.5);
  // series2.add(25, 0.6);
  // series2.add(40, 0.8);
}
final XYSeriesCollection dataset = new XYSeriesCollection();

dataset.addSeries(series1);
dataset.addSeries(series2);
//final XYSeriesCollection dataset1 = new XYSeriesCollection(series2);
//JFreeChart chart = null;

chart  = ChartFactory.createXYLineChart("Xenon Concentration and Poisoning",
        "Time(Hrs)", "Xenon",(XYSeriesCollection) dataset, PlotOrientation.VERTICAL, true, true, false);

ChartPanel frame = new ChartPanel(chart);

       GraphPanel.add(frame,BorderLayout.CENTER);
       GraphPanel.validate();
 }

your help will be highly appreciated
thanks

[mtyoung]

not related to jFreeChart

 String tmp = (String) Xenon.getValueAt(i, 1);

use Double.toString() instead of (String)