Once upon a time (long ago ... and even more recently), I was asked to revise the spreadsheet described here so as to include more than just two assets (called, in the original spreadsheet, "Stocks" and "Bonds").

Hence, the spreadsheet below (which you can download by RIGHT-clicking on the picture and choosing Save Target):

In this spreadsheet, you can:

• Choose four from a dozen (or more) asset classes.
• Insert additional monthly returns (for your favourite stock or index) by downloading the data from Yahoo, like so:

Enjoy