Help with charting values over time

Hello all,

I've used the search function, dug through posts, but I can't seem to figure out how to do this. Here is what I'm trying to do:

Feb-02 Jun-02 12
Aug-02 Sep-03 49
Aug-02 Feb-04 150
Nov-02 Aug-03 28
Jan-03 Jan-05 110
Jan-03 Jan-04 41

I need to make a chart showing the total units per quarter/year. The X-axis would be the quarter (IE Q1-2002, Q2-2002, etc.) and the Y-axis would be the total units active during that period. (Quarters are calendar year based, IE Jan-Mar, Apr-Jun, etc)

The trouble I'm having is figuring out the formula to add/subtract as necessary. The units need to be added on the date of the first column, and subtracted on the date in the second column.

Been trying for a few days, and I just can't get it to work.

Can anyone help?

Thank you very much in advance!