Android : achartengine, dataset.addSeries nullpointerException

List title data -> [input,output] List values data -> [2000,1000]

I try draw chart.. but dataset.addSerires has nullpointerException. I don't know why Why I get an exception...

this is code from the partial exception

for (int i = 0; i < title.size(); i++) {  
                 XYSeries series = new XYSeries(title.get(i));  
                 double v = values.get(i); 
                 series.add(v,v); 
                 System.out.println(v);
                  try{
                     dataset.addSeries(series);  
                     }catch(Exception e){
                       Log.v("Exception",e.toString()+"w");
                      }
                                 }