Changing color of a single gridline on a chart

[Anise Ahmed]

I need to change the color of a single grid line on a
chart. This is currently not allowed (at least I am not
able to). When you change color all the grid lines take
the same color. I need to change the color of a single
grid line. Please help if you know the answer.

 

[Debra Dalgleish]

You can't colour one gridline, but Jon Peltier has instructions for
adding a horizontal line to a chart:

http://www.geocities.com/jonpeltier/Excel/Charts/ComboCharts.html#AddLine