don't understand this offset

[rxp409]

Hi
I am working towards a chart with a scroll bar in excel 2002 showing 32 data entries/rows. My data range is I46:O178 with the labels in A46:A178. I've been following this tutorial Dynamic Charting Formulas in Excel plus some other sites for the control toolbox scroll bar. The dynamic named range bit is the easy part, for my first set of data in column I i have created a named range

=OFFSET(Sheet1!$I$46,COUNTA(Sheet1!$I$46:$I$178)-1,0,-32,1)

I understand the offset function, but why is the COUNTA() function in the argument for offsetting rows? So far as i understand i want the range to start at I46 then for the row offset surely that should be dictated by the cell linked to the scroll bar (which i haven't yet got to work).

Supposing that the scroll bar is linked to I183, i have tried

=OFFSET(Sheet1!$I$46,COUNTA(Sheet1!$I$46:$I$178)+Sheet1!$I$183*-1,0,-32,1)

as per the tutorial, and applied that to the other data ranges and labels, but doesn't seem, to change the chart.

 

[rxp409]

OK i've got the scroller to work through using the offset function as i understand it. But out of curiosity still don't get why the counta would be used in that argument.