The problem you are going to have is setting a benchmark and then assigned a “weight’ to each individual stock. For example if AAPL sells at $10/Share and GOOG sells at $30/share, how do assign values as part of the valuation. Is GOOG always going to be 75% of the value of the portfolio?
This is how the Dow-Jones index works. Each of the 30 stocks has a relative weighting which traces back to the original weight of each stock in the original average as calculated in what, 1892?
You could chart the aggregate value of your portfolio every day using the change is total value or the change in % of value without too much trouble and I think Yahoo finance allows you to set up a portfolio with total value by stock (say, 1000 of AAPL = $10,000) so that you can track the sums daily.
You also have to consider what happens if you have a stock dividend declared or you purchase 100 more shares of GOOG or sell the same, how would that affect your calculations?
I would think that something like Microsoft Money would have this covered. Check around for existing software before trying to create this from scratch.
SRM