Ragdyer:
Sumproduct is just what I was looking for. What an elegant solution! Thank
you.

MH:
The data is exported from a sql database. The database is currently under
development and doesn't provide the calculation and graphing functions that I
require. Yet!

I should probably start a new thread for this, but I'm still stumped by the
date issue. Dates are stored as a dd/mm/yyyy date string. How can I filter
out dates within a particular month for use within calculations, without
constructing complicated if and if< functions. Not only does this become
unwieldy, but is difficult when data covers multiple years.

Thank you for your help and suggestions.