Using formula result for data series bound
 

I'm trying to make a template that creates a bunch of plots from a data file
of unknown length. In order to do this properly, I need to dynamically set
my data series range for the charts. For example my current file plots two
series by executing:
=SERIES("data",Sheet1!$A$3:$A$35,Sheet2!$A$3:$A$35 ,1)
Here I plot the data from Sheet 2:ColumnA:Rows3-35 with respect to the data
from Sheet1:ColumnA:Rows3-35.

What I really want to do is plot the data from row 3 to row (3 + a number
resulting from a formula in another cell). In the original case this number
is 32, hence I go to row 35. My first attempt, so you can better see what I
mean is:
=SERIES("data",Sheet1!$A$3:$A$(3+$B$1),Sheet2!$A$3 :$A$(3+$B$1),1)
Here I want to go down to row 3+$B$1 in column A of each sheet. This syntax
is invalid. Does anyone know the correct way to do this?

Thanks so much!