Scale of a Xaxis NullPointerException

I don't know why i have a NullPointerException when i use the jpeg encoder
Is there anaone could help me.....
It is always the same problem with the scale of the xaxis on a point chart


DataAxisProperties x=new DataAxisProperties();
    DataAxisProperties y=new DataAxisProperties();
    
    
    
    //DataAxisProperties datatry = (DataAxisProperties)axisProperties.getXAxisProperties();
    x.setNumItems(this.intervalle);
    x.setUserDefinedScale(0,24);
    x.setRoundToNearest(0);
    
    //DataAxisProperties dataAxisProperties= (DataAxisProperties) axisProperties.getYAxisProperties();
    //calcul de la meilleur echelle possible
    int n=(int)Math.ceil(new Double(String.valueOf(this.valeurMax/100)).doubleValue());
    y.setNumItems(n+2);
    y.setUserDefinedScale( 0, 100 );
    y.setRoundToNearest( 0 );
 
    
    AxisProperties axisProperties= new AxisProperties((LabelAxisProperties)x,y);
    
    //ligne pour le cadrillage
    ChartStroke xAxisGridLines= new ChartStroke( new BasicStroke( 0.5f ), Color.GRAY );
    axisProperties.getXAxisProperties().setGridLineChartStroke( xAxisGridLines );
    axisProperties.getXAxisProperties().setShowGridLines( AxisTypeProperties.GRID_LINES_ONLY_WITH_LABELS);
    axisProperties.getXAxisProperties().setShowTicks( AxisTypeProperties.TICKS_NONE );
    axisProperties.getXAxisProperties().setPaddingBetweenAxisAndLabels(5.0f);
   
    
    //generation finale
    AxisChart axisChart= new AxisChart( dataSeries, chartProperties, axisProperties,null, 700, 500 );
    
   
   // axisChart.xAxis=new XAxis(axisChart,this.intervalle);
    
     try{
          JPEGEncoder13.encode(axisChart,1.0f,new FileOutputStream(prop.getProperty("GRAPHE")+chemin+".jpg"));
            }

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