Hi,

You can not format individual gridlines. But you can work around the
problem by adding extra data series to plot the lines.
Here is a collection of examples.

http://peltiertech.com/Excel/Charts/RunChtLines.html
http://peltiertech.com/Excel/Charts/AddLine.html
http://tushar-mehta.com/excel/charts...nes/index.html
http://www.andypope.info/charts/averageline.htm

As for changing the cells when certain values are entered you need to
look at Conditional Formatting under the Format menu. For an explanation
see,
http://www.contextures.com/xlCondFormat01.html

Cheers
Andy

Jennie Wren wrote:
I want to show the minium and maximum temp lines on a chart to show when the
temps have gone over or under - clearly. How can I change ONE gridline to
another colour/style?

Also in the spreadsheet I would like a similar thing. Every time I place a
number in that is over or under certain amounts - I would want that figure to
automatcially show in a colours or flash etc.

--

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