How do I put a marker line on an excel chart (i.e. on y axis)?

[ds8854]

How can I put a merker line on an exel line chart (a horizontal line from one
value on the y axis to show when values exceed a certain level on y axis)?

 

[Andy Pope]

Hi,

You can find details of adding datum lines here
http://www.andypope.info/charts/averageline.htm

Cheers
Andy