Adding a second line to a graph in excel via c#

Question

I want to create a simple line on a graph in excel that I have generated with code. I add the data to an excel sheet and can generate a single graph of that data. my code to generate the graph is

int cRowCount = worksheet.UsedRange.Rows.Count;
        Range employeeNames = worksheet.Range["A2:A" + cRowCount];
        Range kpiValues = worksheet.Range["E2:E" + cRowCount];
        Chart kpiChart = (Chart)workbook.Charts.Add(misValue, misValue, 1, misValue);
        kpiChart.SetSourceData(employeeNames, misValue);

        kpiChart.ChartType = XlChartType.xlLineMarkers;

        var seriesX = (Series)kpiChart.SeriesCollection(1);

        seriesX.XValues = employeeNames;
        seriesX.Values = kpiValues;
        
        kpiChart.Legend.Clear();
        workbook.Sheets["KPI Export"].Move(workbook.Sheets[1]);
        kpiChart.HasTitle = false;
        workbook.Sheets[2].name = "KPI chart";

        // save the application  
        workbook.SaveAs(tbxExcelSave.Text + "\\" + tbxName.Text, XlFileFormat.xlOpenXMLWorkbook,
        misValue, misValue, misValue, misValue, XlSaveAsAccessMode.xlExclusive, misValue, misValue, misValue, misValue, misValue);

        // Exit from the application  
        writeToApp.Quit();
        GC.Collect();

The graph looks as follows.

Graph of all values I now want to use the final data point to create a horizontal line in the same graph as the final data point is the average of all the data points. With this I will be able to see what is above or below the "average" of the data.

Really all I need to do is add a simple horizontal line to the graph at the Y coordinate of the final value. Any assistance would be greatly appreciated.