This is a great idea. I use an online financial tracker, however like you pointed out this only tell me what i have spent previously and aggregates it. Its useful data however i had to create my own personal excel financial tracker to help my anticipate my future spending in the month but i spent more time that i would like to updating it. Looking forward to seeing the end product, give me a shout on twitter once its up and ready! can also stick it on PH when you are ready! all the best!