Assuming you are dealing with a large number of records I still might be
inclined to use a pivot table. You will want to combine all of the data into
one master spreadsheet. In the master spreadsheet you will need to add a
helper column to indicate the period so that you data will look something
like this...
Client Code/Client Name/Sales/Month
Now you can create a pivot table with this data. Place you cursor anywhere
in the middle of the data and select Data -> Pivot Tables. A wizard will open
up but you can probably just select finish as the default are most likely
what you want. A new Sheet will be created. Place your Client Code / Client
Name in the left hand column. Place the Periods across the top and your sales
in the center. As time goes by you can just keep on inserting more periods
into the source data and updating your pivot table to include the new
periods. Also there are lots of different formatting options to make your
data look the way you want it to.