Storing a value for repeated use

I have a database where I input purchases and sales of two (or more)
stocks/shares
There may be one or more transactions in a day or none at all
I also input the daily price - date and price
I want my report to calculate the running balance of the stock holding and
the current value each day so that I know how much my portfolio is worth each
day
I have made reasonable progress but my problem is that despite having prices
for every working day the report only shows valuations for dates where there
is a purchase or sale transaction on a share - say buy on 4/12/07 and sell on
7/12/07.
I want the report to show the 4/12/07 value on 5/12/07 (4/12/07 stock
holding times 5/12/07 price) & 6/12/07 (4/12/07 stock holding times 6/12/07
price) i.e. until there is a transaction that changes the stock holding.
Is anyone able to give me any ideas?

Thanks