Take a look to the following:
'Set the culture information for the current thread to English before instantiating Excel.Without this line of code nothing works. It's a bug of MS
System.Threading.Thread.CurrentThread.CurrentCulture = System.Globalization.CultureInfo.CreateSpecificCulture("en-US"

'or "fr-FR" or ..., this depends on your Excel version
' Start Excel and get Application object.
oExcel = CreateObject("Excel.Application"

oExcel.Visible = True
' Get a workbook.
Dim wbk As Object
wbk = oExcel.Workbooks.Open(Filename:="E:\Documents and Settings\Fotini\Desktop\MyFile.xls", UpdateLinks:=False, ReadOnly:=False)
Dim oSheet As Excel.Worksheet
oSheet = wbk.ActiveSheet
' Add a Chart for the selected data.
Dim oResizeRange As Excel.Range
oResizeRange = oSheet.Range("B7:B22"

.Resize(ColumnSize:=2)
Dim oChart As Excel.Chart
oChart = oSheet.Parent.Charts.Add
Dim oSeries As Excel.Series
With oChart
.ChartWizard(oResizeRange, Excel.XlChartType.xlLine, , Excel.XlRowCol.xlColumns, , , , "Chart Title", "Axis X title", "Axis Y title"

oSeries = .SeriesCollection(2)
oSeries.XValues = oSheet.Range("A7", "A22"

.SeriesCollection(1).Name = "Voie 24%"
.SeriesCollection(2).Name = "Voie 25%"
.Location(Excel.XlChartLocation.xlLocationAsObject, oSheet.Name)
End With
' Free any references.
oChart = Nothing
oResizeRange = Nothing
oSheet = Nothing
wbk = Nothing
End Sub