adding an average line in excel charts

How to add an average line to a chart in Excel. I am using Excel 2007 but the same applies to earlier versions. I am sure that this is just one way to do it and would love to hear any other ways.

[adrianagiorgica]

That was really helpful after searching lots of other websites and didn't understand how they do it!
Many thanks,

[mattoberle]

Thanks! This is so much simpler than other ways I have seen to do this.

[monikakalra]

Thanks a lot for this. This is really easy and simple way of doing it. brilliant

[jaguar]

you have really helped me alot. Thank you so much

[karoltoja]

It might sounds silly, but I couldn't invent how to do it, thx for this film :)