Example:

//Instantiating a Workbook object
Workbook workbook = new Workbook();
//Adding a new worksheet to the Excel object
int sheetIndex = workbook.getWorksheets().add();
//Obtaining the reference of the newly added worksheet by passing its sheet index
Worksheet worksheet = workbook.getWorksheets().get(sheetIndex);
worksheet.getCells().get("A1").putValue(50);
worksheet.getCells().get("A2").putValue(100);
worksheet.getCells().get("A3").putValue(150);
worksheet.getCells().get("A4").putValue(200);
worksheet.getCells().get("B1").putValue(60);
worksheet.getCells().get("B2").putValue(32);
worksheet.getCells().get("B3").putValue(50);
worksheet.getCells().get("B4").putValue(40);
        
//Adding a chart to the worksheet
int chartIndex = workbook.getWorksheets().get(0).getCharts().add(ChartType.COLUMN, 3, 3, 15, 10);
Chart chart = workbook.getWorksheets().get(0).getCharts().get(chartIndex);
chart.getNSeries().add("A1:a3", true);
chart.getNSeries().get(0).getTrendLines().add(TrendlineType.LINEAR, "MyTrendLine");
Trendline line = chart.getNSeries().get(0).getTrendLines().get(0);
line.setDisplayEquation(true);
line.setDisplayRSquared(true);
line.setColor(Color.getRed());