Why not just add horizontal line to your chart?
Andy Pope has a tutorial on adding an average line to a line chart:
http://www.andypope.info/charts/averageline.htm
I have a tutorial on adding horizontal lines to an XY chart:
http://processtrends.com/pg_charts_horizontal_line.htm
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"Yendor" <(E-Mail Removed)> wrote in message
news:EFC81142-4165-48A7-A92C-(E-Mail Removed)...
>I am attempting to highlight a single threshold gridline in an Excel chart.
> I am able to highlight all of the chart gridlines, but not just the one I
> want. I have MS Office XP. Any suggestions?