calculate total every so often for basis of graph

[wplate]

I would like to start building a graph layout (using chart builder from briandunning.com) that displays historical payment data.

I would like to calculate subtotals in two-week increments then store each result in a field that will be used by the graphing engine.

I suppose I need a calculation field that does a Sum calculation on my payment field, but I can't figure out how the calculation would only use the records that fall into the date range I set.