Will there be multiple records in items for the same item each month? Assuming that the answer to that question is yes....
First define a calculation field, cItemCount, in Items: Sum ( LineItems::Quantity )
Then define a summary field in items that computes the total of cItemCount.
Also define cMonth as a calculation field returning a date: Date - Day ( Date ) + 1
This returns the date of the first day of the same month to provide a common value for grouping your records by date.
Use List view layout to show the records from Items. Replace the body of the layout with a sub summary layout part "when sorted by" id. Put the summary field in this sub summary layout part plus any other fields from items that you want.
Sort your records first by cMonth and then by id to properly group your records for this summary report.
One more thing, The cMonth field will have year stored right? Otherwise won't it will start adding the last months records of the same month?
It's a date field so it includes month, day and year, but the value for day is always 1. And thus this will not combine data from the same month but different years.