Perform Action Without Switching To Sheet

Select triggers Activate event
Either add

Private Sub Chart_Activate()
    Application.EnableEvents = Fase
    Sheets("A. Data Sheet").Select
    ActiveSheet.Outline.ShowLevels RowLevels:=0, ColumnLevels:=2
    Sheets("B. Graph Sheet").Select
    ActiveChart.ChartArea.Select
    Application.EnableEvents = True
End Sub

Or no Select/Activate

Private Sub Chart_Activate()
    Sheets("A. Data Sheet").Outline.ShowLevels RowLevels:=0, ColumnLevels:=2
End Sub