Calculate total numbers based on percentages

[FM1]

Hey everyone. I have a spreadsheet with the following information:

Column A - Name
Column B - Allocation
Column C - Start Date
Column D - End Date

I need to work out the total number FTE (full time equivalent) resources working every month and present it in a chart. Example - if John Smith has an allocation of 0.8 and is working from 01/01/09 to 01/02/09 and Tim Bob has an allocation of 0.2 and is working from 01/01/09 to 01/02/09 that will count has one full time equivalent resource for that period.

I have attached a sample spreadsheet. Thanks in advance.