question on horizontal line on chart
 

Excel 2003 :)
Ok, I'm able to put a horozontal line on my chart
but now I'm trying to make it change...
I have a chart (standard clustered column)that shows YTD values from several
sources with a horizontal line showing the average of them all.
(instructions for adding line from peltiertech.com - thanks Jon!)
Here's my problem.
These are year to date numbers so they change monthly and so does the
average.
In the x,y range in my worksheet for the horizontal line, I tried linking
the number to the average, and it appears to work - the numbers for 0 and 1
change when the average changes... but the value doesn't change on my chart.