query to average weekly data

Experts,
I'm trying to create a date parameter query for a line chart that:
1) Based upon the date range create a line chart for each week ending on a Friday where it returns an average for the week. For example, a date range of 6/1/08 - 6/30/08 would return the following:
06/08/08      750
06/13/08      655
06/20/08      600
06/27/08      825
I started with this:

SELECT tblGrossStrokesPerHour.GSPHDate, tblGrossStrokesPerHour.GrossStrokesPerHour, tblGrossStrokesPerHour.LineCode
FROM tblGrossStrokesPerHour
WHERE (((tblGrossStrokesPerHour.GSPHDate) Between [Start Date] And [End Date]) AND ((tblGrossStrokesPerHour.LineCode)=8))
ORDER BY tblGrossStrokesPerHour.GSPHDate;

I attached the a database
 
gsph.zip