Adding a line to a chart

I've attached my first attempt at showing data using a chart. I am tracking dart board usage at a tournament. Time of day is across the bottom, and the number of boards needed is the vertical. I would like to add a line that represents the number of boards I actually have (which is 38) to make it easier to see what parts of the day I am in trouble. I found how to change from a line every 10 to a line every 5, but I can't figure out how to add an additional line at 38.

Thanks in advance!

I would start by trying one of these techniques: http://peltiertech.com/Excel/Charts/AddLine.html