Expanding Chart

Hello,
I copied an expanding chart from a suggestion from this site but I would like to revamp it so that when you add the next set of data points it drops the first set of data points.

For example the chart charts a5:b23 and when I enter information in A24 and B24 I want the chart to chart from A6:b24.

Cht_X_Values=OFFSET(Sheet1!$A$5,1,0,COUNT(Sheet1!$A$6:$A$2004),1)
Cht_Y_Values=OFFSET(Sheet1!$B$5,1,0,COUNT(Sheet1!$A$6:$A$2004),1)
Chart series formula:=SERIES(Sheet1!$B$5,FUNCHRT1.XLS!Cht_X_Values,FUNCHRT1.XLS!Cht_Y_Values,1)

Thanks in advance for your knowledge and help. It is greatly appreciated.
Michelle

Here is an explaination of last 12 periods, which you can adapt.
http://peltiertech.com/Excel/Charts/DynamicLast12.html