Hi,

You can not format individual grid lines.
You can add a dummy series to act like a grid line and format that as
required.

Jon Peltier has an explanation for creating arbitrary grid lines that
you can adapt.
http://peltiertech.com/Excel/Charts/...Gridlines.html

Or here are some explanations of adding lines to charts.
http://peltiertech.com/Excel/Charts/AddLine.html
http://tushar-mehta.com/excel/charts...nes/index.html
http://www.andypope.info/charts/averageline.htm

Cheers
Andy

Dan deGrassi wrote:
How do I highlight (e.g., bold) just 1 particular y-axis gridline? I want to
make it stand out as a threshold.

--

Andy Pope, Microsoft MVP - Excel
http://www.andypope.info