Web Development

I have stitched together the beginnings of a mortgage calculator, but need to add features which are beyond me right now. Current: http://s100053443.onlinehome.us/loan-calculator.php
Need to:
1) make additional principle payment field calculate into the graph and the summary table showing difference between the extra paymment and no extra payment.
2) want to make the amortization table yearly instead of per period.
Can you help???
Thanks!