public void refresh(Map<String, List<String[]>> viewData) {
   this.prviewData = viewData;
   initChartData(viewData);
   jfreeChartComp.setChart(computeChart());
   jfreeChartComp.forceRedraw();
 }
 /** create a chart with empty dataset,so as to clear the blocking key chart. */
 public void clearChart() {
   JFreeChart jfreechart =
       createBarChart("Number of rows", new HistogramDataset(), false); // $NON-NLS-1$
   jfreeChartComp.setChart(jfreechart);
   jfreeChartComp.forceRedraw();
 }