//How to Set Category Axis

//Your local test path
String path = @"";

//Create a new workbook
Workbook workbook = new Workbook();
Worksheet worksheet = workbook.Worksheets[0];
worksheet.Name = ("CHART");

// Add a chart to the worksheet
int chartIndex = worksheet.Charts.Add(ChartType.Column, 8, 0, 20, 10);
Chart chart = worksheet.Charts[chartIndex];

//Add some values to cells
worksheet.Cells["A1"].PutValue("Sales");
worksheet.Cells["A2"].PutValue(100);
worksheet.Cells["A3"].PutValue(150);
worksheet.Cells["A4"].PutValue(130);
worksheet.Cells["A5"].PutValue(160);
worksheet.Cells["A6"].PutValue(150);
worksheet.Cells["B1"].PutValue("Days");
worksheet.Cells["B2"].PutValue(1);
worksheet.Cells["B3"].PutValue(2);
worksheet.Cells["B4"].PutValue(3);
worksheet.Cells["B5"].PutValue(4);
worksheet.Cells["B6"].PutValue(5);

//Some values in string
String Sales = "100,150,130,160,150";
String Days = "1,2,3,4,5";

//Set Category Axis Data with string
chart.NSeries.CategoryData = "{" + Days + "}";
//Or you can set Category Axis Data with data in cells, try it!
//chart.NSeries.CategoryData = "B2:B6";

//Add Series to the chart
chart.NSeries.Add("Demand1", true);
//Set value axis with string 
chart.NSeries[0].Values = "{" + Sales + "}";
chart.NSeries.Add("Demand2", true);
//Set value axis with data in cells
chart.NSeries[1].Values = "A2:A6";

//Set some Category Axis properties
chart.CategoryAxis.TickLabels.RotationAngle = 45;
chart.CategoryAxis.TickLabels.Font.Size = 8;
chart.Legend.Position = LegendPositionType.Bottom;

//Save the workbook to view the result file
workbook.Save(path + "Output.xlsx");