Is there a website that will generate a single-line chart, graphing the performance of a collection of stocks over time?

Whenever I enter multiple stock symbols into Google Finance or Yahoo Finance, I get a chart that contrasts the companies against each other. I’m looking for a similar chart that shows the combined performance of the companies. For example, an index that consists only of AAPL, GOOG, AMZN, NFLX, and TIVO, graphed as a single line over time.