A composite stock chart

This will probably be a challenge.  Would it be possible to create a composite "index" or "average" of different stocks.  

Say, for example I am analysis several mining stocks like BHP, RIO, VALE, TECK FCX.

Sometimes its easier to see what's going on by comparing a stock chart to a customized index (market weighted) or average (stock price average) to see if the stock is being influenced by systematic forces (pertaining to companies in the entire industry) or idiosyncratic forces (which are peculiar to the company itself)

Such a tool would be very useful.  Of course, it might be difficult to engineer.