Hi,
You can not colour an individual gridline.
But here are some examples of how to add a datum line to your chart.
http://www.andypope.info/charts/averageline.htm
http://peltiertech.com/Excel/Charts/AddLine.html
http://tushar-mehta.com/excel/charts/straight_lines/index.html
http://processtrends.com/pg_charts_horizontal_line.htm
Cheers
Andy
> Is it possible to change the color of just one axis line on a chart? It
> would be nice because then we could mark in red which would allow us to see
> target values we are shooting for and then see how the actual results measure
> up.

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