how to plot line graph by reading x and y axis from text file?

hello
In the below code we are able to read y-axis but not able to read x-axis on the graph.. can tel me how to read x-axis on the graph.
thanks in advance

import java.io.*;
import java.util.*;//Scanner is included
import org.jfree.chart.*;
import org.jfree.chart.plot.PlotOrientation;
import org.jfree.data.category.DefaultCategoryDataset;
import org.jfree.data.general.DefaultPieDataset;
import org.jfree.data.xy.*;
import org.jfree.data.*;
public class Plot_1
{
static Scanner scanner;
static XYSeries series;
//static File file;
public static void main(String[] args)
{
plotResult();
}//end of main
//**********
//Method that reads in an external file with numbers (doubles)
//and plots the content in a line chart using JFreeChart

public static void plotResult() {
//Read in the file "calculation.txt" using Scanner and call hasNextDouble on it

try {
int index = 0;
File file = new File("data.txt");
series = new XYSeries("Pk");
scanner = new Scanner(file);
// Scanner scanner = new Scanner(file);
while (scanner.hasNextDouble()) {
series.add(index++,scanner.nextDouble());
}
scanner.close();
}
catch (FileNotFoundException e) {
e.printStackTrace();
}
//Round off the doubles to two decimals - not done yet
//Plot the line chart
/*

XYSeries series = new XYSeries("Pk");

series.add(1, 0.8201791789916324 );

series.add(2, 0.9727097916233601 );

series.add(3, 0.9949972829175494);

*/
XYDataset xyDataset = new XYSeriesCollection(series);
JFreeChart chart = ChartFactory.createXYLineChart

("Temp = 85", "k", "Pk",xyDataset, PlotOrientation.VERTICAL, true, true, false);

ChartFrame frame1=new ChartFrame("XYLine Chart",chart);

frame1.setVisible(true);

frame1.setSize(600,400);
}//End of method
//***************
}//end of class

in data.txt has following values
x y
1 20
2 30
4 50