Have set up a spreadsheet which shows the start date, duration and value of
projects. Each may have a different start date (month/year).

I would like to produce a line graph which plots the cumulative income
generated per month.

Name Start Date Duration (months) Value
A 06/2007 3 6,000
B 03/2007 10 180,000
D 08/2007 24 240,000
C 02/2008 6 24,000

I'm thinking in order to create the graph I need a table populated along the
lines of:
=
02/2007 0
03/2007 18,000 18,000
04/2007 18,000 18,000
05/2007 18,000 18,000
06/2007 18,000 + 2,000 20,000
07/2007 18,000 + 2,000 20,000
08/2007 18,000 + 2,000 + 10,000 30,000
09/2007 18,000 + 10,000 28,000
Etc
02/2008 10,000 + 6,000 16,000
Etc

Any ideas using functions, lookups, pivots etc?? (MS Project is not an
alternative for us in this instance!)

Thanks in advance for you help.