There should be a more direct solution that just specifies the right format, etc. straight away, but this fixes the problem as you've described it, and because we're using VB.Net, I've removed the unneeded Missing references. (I guess you converted this from a C# sample.)
Private Sub DisplayGraph()
Try
Dim xlApp As Excel.Application
Dim xlWorkBook As Excel.Workbook
Dim xlWorkSheet As Excel.Worksheet
xlApp = New Excel.ApplicationClass
xlWorkBook = xlApp.Workbooks.Add()
xlWorkSheet = xlWorkBook.Sheets("sheet1")
xlApp.Visible = True
'Adding Data..
xlWorkSheet.Cells(1, 1) = 2
xlWorkSheet.Cells(1, 2) = 1
xlWorkSheet.Cells(2, 1) = 3
xlWorkSheet.Cells(2, 2) = 2
xlWorkSheet.Cells(3, 1) = 2
xlWorkSheet.Cells(3, 2) = 5
xlWorkSheet.Cells(4, 1) = 4
xlWorkSheet.Cells(4, 2) = 7
xlWorkSheet.Cells(5, 1) = 8
xlWorkSheet.Cells(5, 2) = 2
xlWorkSheet.Cells(6, 1) = 5
xlWorkSheet.Cells(6, 2) = 3
Dim oChart As Excel._Chart
oChart = xlWorkBook.Charts.Add()
oChart.ChartWizard(xlWorkSheet.Range("A1", "B6"), _
Excel.XlChartType.xlXYScatterLines, _
PlotBy:=Excel.XlRowCol.xlRows)
oChart.PlotBy = Excel.XlRowCol.xlColumns
Catch ex As Exception
MessageBox.Show(ex.Message)
End Try
End Sub