I learned how to place a horizontal line on my chart at www.peltiertech.com
(thanks Jon!)
but how do I make the value change? I want it to show the YTD average so it
changes monthly. I tried linking the two cells that list the value to the
cell where the average is calculated, and it changes in the cells but not on
the chart.

0 95% <----
1 95% <---- these two cells (with 95% linked to cell where the average is
calculated, and it changes in these two cells, but doesn't change on the
chart.