Powerpoint chart not updating if you close Excel workbook

I am using VBA to update a powerpoint presentation chart in Office 2010, as illustrated in the Microsoft article http://msdn.microsoft.com/en-us/library/gg980939(v=office.14).aspx. Though the Excel data table is indeed updated, the chart does not update if I close the Excel worksheet by calling wb.Close at the end. In this case what code may I use to update the graph/chart while closing the Excel file?

The code I used is below:

    Dim shp As Shape
    Set shp = ppSlide.Shapes(StrainChart)
    Dim cht As Chart
    Set cht = shp.Chart

    Dim wb As Excel.Workbook
    Dim ws As Excel.Worksheet

    cht.ChartData.Activate
    Set wb = cht.ChartData.Workbook
    Set ws = wb.Worksheets(1)

    ws.Range("B2").Value = dMinThick 'max thickness
    ws.Range("C2").Value = dMaxThick 'min thickness

    ws.Range("B3").Value = dMinMajor 'max thickness
    ws.Range("C3").Value = dMaxMajor 'min thickness

    ws.Range("B4").Value = dMinMinor 'max thickness
    ws.Range("C4").Value = dMaxMinor 'min thickness

    wb.Close