Please try this attached fix with the following sample code:
Excel excel = new Excel();
excel.Open("d:\\test\\prisa control chart.xls");
excel.Worksheets.Add(SheetType.Chart);
int chartIndex = excel.Worksheets[4].Charts.Add(ChartType.StockHighLowClose, 0, 0, 0, 0);
Chart chart = excel.Worksheets[4].Charts[chartIndex];
chart.NSeries.Add("'Data Table'!B7:N7", false);
chart.NSeries.Add("'Data Table'!B6:N6", false);
chart.NSeries.Add("'Data Table'!B8:N8", false);
chart.PlotArea.Area.ForegroundColor = Color.Silver;
chart.NSeries.CategoryData = "'Data Table'!B4:N4";
chart.CategoryAxis.CategoryType = CategoryType.CategoryScale;
chart.CategoryAxis.AxisBetweenCategories = false;
chart.CategoryAxis.AxisLine.Style = LineType.Dash;
chart.CategoryAxis.AxisLine.Weight = WeightType.MediumLine;
chart.ValueAxis.MajorUnit = 0.25;
chart.ValueAxis.MinorUnit = 0.05;
chart.MajorGridLines.Style = LineType.Dot;
chart.Title.Text = "Prisa 945 Waist";
chart.Title.TextFont.IsBold = true;
chart.Title.TextFont.Size = 12;
chart.Legend.Position = LegendPositionType.Top;
chart.NSeries[0].Line.IsVisible = true;
chart.NSeries[0].Line.Color = Color.Blue;
chart.NSeries[0].Line.Weight = WeightType.MediumLine;
chart.NSeries[0].MarkerStyle = ChartMarkerType.None;
chart.NSeries[0].Smooth = true;
chart.NSeries[0].Name = "='Data Table'!A7";
chart.NSeries[1].Line.IsVisible = true;
chart.NSeries[1].Line.Color = Color.Red;
chart.NSeries[1].Line.Weight = WeightType.MediumLine;
chart.NSeries[1].MarkerStyle = ChartMarkerType.None;
chart.NSeries[1].Smooth = true;
chart.NSeries[1].Name = "='Data Table'!A6";
chart.NSeries[2].Line.IsVisible = true;
chart.NSeries[2].Line.Color = Color.Blue;
chart.NSeries[2].Line.Weight = WeightType.MediumLine;
chart.NSeries[2].MarkerStyle = ChartMarkerType.None;
chart.NSeries[2].Smooth = true;
chart.NSeries[2].Name = "='Data Table'!A8";
excel.Worksheets[4].Zoom = 75;
Could you please tell me how you created the UCL and LCL line? I don't find them in Excel chart setting.