I have managed to achieve the desired cascading of combo boxes using some VB
Private Sub OpCoName_AfterUpdate()
Me!Combo5.RowSource = "SELECT Distinct [Year] FROM [qryPortAnalysis2A] WHERE [OpCo_Id]=" & Me!OpCoName & " ORDER BY [Year];"
I then linked the combo boxes to my chart using the Link Child/Master properties and the graph toggled between datasets correctly.
However, when I close the form and re open it a pop-up box appears asking for parameter values of the fields in the Link Child Field property of the chart.
Why is this? I have tried to populate these fields using onload
Private Sub Form_Load()
[Forms]![frmPortAnalysis2]![OpCoName] = 1
[Forms]![frmPortAnalysis2]![Combo5] = 2005
But it still does not work - I feel like I'm alomst there!