@airportyh,
You may use Series.DataLabels.NumberFormat property to get this information as demonstrated in the following sample code. Search for the comments //NOTE THIS LINE.
//Instantiating a Workbook object
Workbook workbook = new Workbook();
//Adding a new worksheet to the Excel object
int sheetIndex = workbook.Worksheets.Add();
//Obtaining the reference of the newly added worksheet by passing its sheet index
Worksheet worksheet = workbook.Worksheets[sheetIndex];
//Adding a sample value to "A1" cell
worksheet.Cells["A1"].PutValue(50);
//Adding a sample value to "A2" cell
worksheet.Cells["A2"].PutValue(100);
//Adding a sample value to "A3" cell
worksheet.Cells["A3"].PutValue(150);
//Adding a sample value to "B1" cell
worksheet.Cells["B1"].PutValue(4);
//Adding a sample value to "B2" cell
worksheet.Cells["B2"].PutValue(20);
//Adding a sample value to "B3" cell
worksheet.Cells["B3"].PutValue(50);
//Adding a chart to the worksheet
int chartIndex = worksheet.Charts.Add(ChartType.Column, 5, 0, 25, 5);
//Accessing the instance of the newly added chart
Chart chart = worksheet.Charts[chartIndex];
//Adding NSeries (chart data source) to the chart ranging from "A1" cell to "B3"
int index = chart.NSeries.Add("A1:B3", true);
var series = chart.NSeries[index];
Console.WriteLine(series.DataLabels.NumberFormat);//NOTE THIS LINE
//Set the max value of value axis
chart.ValueAxis.MaxValue = 200;
//Set the min value of value axis
chart.ValueAxis.MinValue = 0;
//Set the major unit
chart.ValueAxis.MajorUnit = 25;
//Category(X) axis crosses at the maxinum value.
chart.ValueAxis.CrossType = CrossType.Maximum;
//Set he number of categories or series between tick-mark labels.
chart.CategoryAxis.TickLabelSpacing = 2;
//Saving the Excel file
workbook.Save("book1.xls");