Can I create a dynamic chart with a dynamic number of series?

Is it possible to create a dynamic chart which will accommodate the addition of an unknown number of additional dynamic series using Excel 2007?
That is, I have a db query feeding a table and there will be new rows added each day AND new columns:

............Employee 1.....Employee 2.....Employee 3.....Employee 4.....Employee 5
Jan 1.....10....................5
Jan 2.....9......................1
Jan 3.....12....................11..................8.....................55
Jan 4.....14....................12..................10...................1...................1
Jan 5.....19....................13..................13...................11.................10
Jan 6.....12....................15..................16...................16.................18


As you can see, there is more than one series and new series come on line all the time as new employees are added.

I can easily define the dynamic range:
rowscell=counta(a2:a9999)
columnscell=counta(b1:b9999)
myrange=offset(mysheet!$a$1,0,0,rowscell,columnscell)

However, when I try to use myrange as the graph data range I get a "The formula you typed contains an error" dialog.

Any suggestions?