How can I graph multiple date fields?

[zeeeeek]

I have a table with to important fields: ReceivedDate and ClosedDate and I want to generate a chart which shows the number of items received and the number of items closed per week. If I set the chart to show the counts of ReceivedDate and ClosedDate on change of ReceivedDate then I get an innacurate count, as the count of closedDate for that group is actually the count of items closed in that week which were also opened in that week.
So my question is how can I get a chart which shows accurate counts of each field per week?