How can I display a specified y-constant line on a graph where the input variable is a spike graph?

[Ross h]

I currently have a signal going into a graph which displays the value
over time as a spike graph around zero.  I want to add onto this graph
a line which shows a limit that the signal should fall within at say
+-10. ie a horizontal line added to the graph.  Is there any way that
I can do this.

Thanks

Ross