Editing Charts in Excel 2007 using vb.net

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VB

MS-Excel

chart

Hi,

I am new to this site. I have a question regarding editing graphs using vb.net code. I have a graph named 'ChartModel', I use a code to edit the Chart Title, but it gives me an error that says invalid index. The title will be concatenated though a combo box from the GUI. I searched the site for articles regarding editing graph using vb.net, but I can't find any. I just found articles on how to create graphs. Currently here is my code:

Public Shared Sub FormatCharts(ByVal WB As Excel.Workbook)
        Dim WS As Excel.Worksheet = WB.Sheets(4)
        Dim oChart As Excel.Chart
        Dim xlChart As Excel.ChartObjects = WS.ChartObjects
        Dim myChart As Excel.ChartObject = WB.Charts(1)'this is where the error occurs. I also used WB.Charts("ChartModel")
        oChart = myChart.Chart

        With oChart
            .HasTitle = True
            .Activate()
            .Select()
            If ReportControlForm.txtDupOptModel.Text <> String.Empty Then
                .ChartTitle.Text = "Device Count by " & ReportControlForm.txtDupOptModel.Text
            End If

        End With

    End Sub

I can't figure out on how to point it to the correct graph. Any help is greatly appreciated.