Calculating Profit from Stock Sale

[Joshua]

Hopefully I'm not messing something up.  I tried to post this a while ago but 
it doesn't look like it worked.  Anyway...

In the documentation for buying and selling stock, it says that you need to 
record profit or loss from the sale of stock in the income account for that 
stock.  So, if you make regular monthly purchases of a stock or fund over the 
course of several years, and you periodically sell some shares of a stock or 
fund in order to buy new cars and houses, or pay for fund fees, etc., how do 
you determine the profit from those sales?

The documentation doesn't clearly describe this situation with it's buy and 
sell examples because they only include a single purchase or sale.

Does anyone know how to handle this?