John -

Gridlines can't be individually formatted. You can fake gridlines by adding series
or error bars:

http://peltiertech.com/Excel/Charts/AddLine.html
http://peltiertech.com/Excel/Charts/...Gridlines.html

This won't work in a 3D chart, because you can't make a 3D combination chart.
Instead of a 3D bar or column chart, which look great from a marketing standpoint
but are notoriously difficult to interpret, couldn't you make a clustered bar or
column chart? These can be combined with scatter chart series for the gridlines.

- Jon
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Jon Peltier, Microsoft Excel MVP
Peltier Technical Services
Tutorials and Custom Solutions
http://PeltierTech.com/
_______

John Canino wrote:

I spent a great deal of time today trying to mess with this. I was using a
three variable 3D bar chart for a report. I needed to super-impose a
department average and a company target. This would have been simple if I
could have changed the color of the gridlines representing these two values.
The trendline option, if it would have worked, stayed dimmed no matter what I
tried to do. Thanks for any input.