problem with pie chart

static void Main(string[] args)
{
//Create a Workbook
Workbook workbook = new Workbook();

//Get the first sheet and set its name
Worksheet sheet = workbook.Worksheets[0];
sheet.Name = "Pie Chart";

//Add a chart
Chart chart = null;

chart = sheet.Charts.Add(ExcelChartType.Pie3D);

//Set chart data
CreateChartData(sheet);

//Set region of chart data
chart.DataRange = sheet.Range["B2:B4"];
chart.SeriesDataFromRange = false;

//Set position of chart
chart.LeftColumn = 1;
chart.TopRow = 6;
chart.RightColumn = 9;
chart.BottomRow = 25;

//Chart title
chart.ChartTitle = "Sales by year";
chart.ChartTitleArea.IsBold = true;
chart.ChartTitleArea.Size = 12;

ChartSerie cs = chart.Series[0];
cs.CategoryLabels = sheet.Range["A2:A4"];
cs.Values = sheet.Range["B2:B4"];
cs.DataPoints.DefaultDataPoint.DataLabels.HasValue = true;

chart.PlotArea.Fill.Visible = false;

//Save the file
workbook.SaveToFile("Output.xlsx", ExcelVersion.Version2010);
}

private static void CreateChartData(Worksheet sheet)
{
DataTable dt = new DataTable();
dt.Columns.Add("Year", typeof(string));
dt.Columns.Add("sales", typeof(int));
DataRow row1 = dt.NewRow();
row1[0] = "2002";
row1[1] = 4000;
dt.Rows.Add(row1);
DataRow row2 = dt.NewRow();
row2[0] = "2003";
row2[1] = 6000;
dt.Rows.Add(row2);
DataRow row3 = dt.NewRow();
row3[0] = "2004";
row3[1] = 7000;
dt.Rows.Add(row3);
//export data from datatable to sheet
sheet.InsertDataTable(dt, true, 1, 1);
}