Execute macro automatically after cell update

The following code only executes after I update the relevant cell (U8) if I click anywhere on the sheet.

Any ideas on how I can get it to execute automatically after I update this cell?

Thanks.

Private Sub Worksheet_SelectionChange(ByVal Target As Range)

    Dim celltxt As String
    celltxt = ActiveSheet.Range("U8").Text
    If InStr(1, celltxt, "Calendar Year") Then
            ActiveSheet.ChartObjects("Chart 1").Activate
            ActiveChart.PlotArea.Select
            ActiveChart.SetSourceData Source:=Range("B13:G15")
        Else
            If InStr(1, celltxt, "Half Year") Then
            ActiveSheet.ChartObjects("Chart 1").Activate
            ActiveChart.PlotArea.Select
            ActiveChart.SetSourceData Source:=Range("B13:I15")
        Else
            If InStr(1, celltxt, "Financial Year") Then
            ActiveSheet.ChartObjects("Chart 1").Activate
            ActiveChart.PlotArea.Select
            ActiveChart.SetSourceData Source:=Range("B13:F15")
            
    End If
    End If
    End If


End Sub