How to overlay a symbol on a time series plot

[Yang Haijun <navyang@xxxxxxxxxxxxxxxxxxxx>]

Hi Dear ferreters,

How can I overlay a symbol on a time series plot? 

For example, I have a station temperature with 40 years. I get the mean temperature over 40 
years. Then I plot the time series of temp,

 yes? plot/l=1:40 temp
 yes? let tmean=temp[l=1:40]

Now I want to plot a symbol which represents tmean, on the time series plot. How can I get that?

Thanks a lot.

Haijun Yang