Average overdue per customer

I'm trying to do a scatter chart of overdue invoices to highlight overdue invoices and whether a spefic customer that is not normally a bad payer hasn't paid as well as highlighting bad payers.

x axis =  Days overdue for open invoices

y axis = Customers average days to pay

size = Open invoice amount

I have an invoicehead table and customer table linked on customer ID.

I have calculated the

x axis (Days overdue for open invoices) = DaysOverDue(Open) = IF(View_InvcHead[OpenInvoice]=TRUE(),
TODAY()-View_InvcHead[DueDate],
0)

Size (OpenInvoiceAmount) = SUM(View_InvcHead[InvoiceBal])

What I am struggling with is calculating the Customers average days to pay. This is the average between the duedate and the closed date per customer.

I have tried AverageX but it still seems to calculate it per invoice rather than per customer.

Regards

Richard