Set and Format Major and Minor Gridlines and set the units
Set the major and minor gridlines on value axis. Format the major gridline to 2 points wide and color blue. Format the minor gridline to 0.5 points wide and color light blue. Set the major gridline units to 2, the minor gridline units to 0.5.
Output: MajorMinorGridlinesFormat.xlsx
var workbook = new Workbook();
var worksheet = workbook.Worksheets[0];
worksheet.GetRange("J1:L1").SetValue(new[] { "Series 1", "Series 2", "Series 3" });
worksheet.GetRange("I2:I5").SetValue(new[] { "Point 1", "Point 2", "Point 3", "Point 4" });
worksheet.GetRange("J2:J5").SetValue(new[] { 2, 4, 6, 8 });
worksheet.GetRange("K2:K5").SetValue(new[] { 4, 3, 2, 1 });
worksheet.GetRange("L2:L5").SetValue(new[] { 2, 1, -1, -2 });
var chart1 = worksheet.Charts.Add<BarChart>("A1");
chart1.ChartType = ChartType.Column;
chart1.Size.HeightInches = 2.5;
chart1.Size.WidthInches = 3.5;
chart1.DataSource = "Sheet1!$I$1:$L$5";
chart1.ValueAxis.MajorGridLines = new Gridlines();
chart1.ValueAxis.MinorGridLines = new Gridlines();
chart1.ValueAxis.MajorGridLines.Line.ChartLineType = ChartLineType.SolidLine;
chart1.ValueAxis.MinorGridLines.Line.ChartLineType = ChartLineType.SolidLine;
chart1.ValueAxis.MajorGridLines.Line.WidthPoints = 2;
chart1.ValueAxis.MajorGridLines.Line.Color = Color.Blue;
chart1.ValueAxis.MinorGridLines.Line.WidthPoints = 0.5;
chart1.ValueAxis.MinorGridLines.Line.Color = Color.LightBlue;
chart1.ValueAxis.MajorUnits = 2;
chart1.ValueAxis.MinorUnits = 0.5;
workbook.SaveAs("MajorMinorGridlinesFormat.xlsx");