Series lines not showing up properly in JFreeChart

I am working with this plot where the data and the lines are not showing properly. When it has more data and while zooming it with mouse wheel, the lines are not showing properly.

I tried placing polygon box around series line thinking that the series will show up like the XYTextAnnotation, but it's not working.

import java.awt.Color;
import java.awt.BasicStroke;
import org.jfree.chart.ChartPanel;
import org.jfree.chart.JFreeChart;
import org.jfree.chart.annotations.*;
import org.jfree.data.xy.XYDataset;
import org.jfree.data.xy.XYSeries;
import org.jfree.ui.ApplicationFrame;
import org.jfree.ui.Layer;
import org.jfree.ui.RefineryUtilities;
import org.jfree.chart.plot.XYPlot;
import org.jfree.chart.ChartFactory;
import org.jfree.chart.plot.PlotOrientation;
import org.jfree.data.xy.XYSeriesCollection;
import org.jfree.chart.renderer.xy.XYLineAndShapeRenderer;
import org.jfree.ui.TextAnchor;

public class XYLineChart extends ApplicationFrame {

   public XYLineChart(String applicationTitle, String chartTitle) {
      super(applicationTitle);
      JFreeChart xylineChart = ChartFactory.createXYLineChart(
         chartTitle ,
         "Category" ,
         "Score" ,
         createDataset() ,
         PlotOrientation.VERTICAL ,
         false , true , false);

      ChartPanel chartPanel = new ChartPanel( xylineChart );
      chartPanel.setMouseWheelEnabled(true);
      chartPanel.setBackground(Color.WHITE);
      chartPanel.setPreferredSize( new java.awt.Dimension( 560 , 367 ) );
      final XYPlot plot = xylineChart.getXYPlot( );
      plot.setDomainPannable(true);
      plot.setRangePannable(true);
      plot.setBackgroundPaint(Color.WHITE);
      plot.setDomainGridlinePaint(Color.BLACK);
      plot.setRangeGridlinePaint(Color.BLACK);

//annotations
      XYLineAndShapeRenderer renderer = new XYLineAndShapeRenderer( );
      renderer.setSeriesPaint( 0 , Color.BLUE );
      renderer.setSeriesStroke( 0 , new BasicStroke( 1.0f ) );

      XYTextAnnotation a1 = new XYTextAnnotation("data",(double)9.1,9);
      a1.setTextAnchor(TextAnchor.CENTER_LEFT);
      a1.setPaint(Color.BLUE);
      renderer.addAnnotation(a1, Layer.BACKGROUND);

      XYTextAnnotation a2 = new XYTextAnnotation("data",(double)28.2,60);
      a2.setTextAnchor(TextAnchor.CENTER_LEFT);
      a2.setPaint(Color.BLUE);
      renderer.addAnnotation(a2, Layer.BACKGROUND);

      plot.setRenderer(renderer);
      setContentPane( chartPanel ); 
   }

   private XYDataset createDataset( ) {
      final XYSeriesCollection dataset = new XYSeriesCollection( );
      final XYSeries data1 = new XYSeries( "Nothing" );
      data1.add(8,9);
      data1.add(9,9);
      data1.add(7,9);
      dataset.addSeries(data1);
      final XYSeries data2 = new XYSeries( "Nothing1" );
      data2.add(20,60);
      data2.add(28,60);
      dataset.addSeries(data2);
      return dataset;
   }

   public static void main( String[ ] args ) {
      XYLineChart chart = new XYLineChart("",
         "");
      chart.pack();
      RefineryUtilities.centerFrameOnScreen( chart );          
      chart.setVisible( true ); 
   }
}

In case I am missing anything, please let me know.

We can see text in first series (blue), but the line has no meaning.

By default, XYLineAndShapeRenderer renderers "with both lines and shapes visible." It looks like crowded shapes on a short line are the problem. One approach would be to render only the lines initially:

XYLineAndShapeRenderer renderer = new XYLineAndShapeRenderer(true, false);

Then let the user enable rendering the shapes for one or more series as desired:

 renderer.setSeriesShapesVisible(1, true);

Several approaches are examined here. This example illustrates using JCheckBox, and this example illustrates clicking on the legend to toggle its state.