XSSFSheet sheet = wb.createSheet();
CTChart ctChart = chart.getCTChart();
CTPlotArea plotArea = ctChart.getPlotArea();
CTPieChart pieChart = plotArea.getPieChartArray(0);
//Pie Chart Series
CTPieSer ser = pieChart.getSerArray(0);
// Series Text
CTSerTx tx = ser.getTx();
tx.getStrRef().getStrCache().getPtArray(0).setV(chartTitle);
sheet.createRow(0).createCell(1).setCellValue(chartTitle);