Average line fully across one bar in bar chart?

Hello, I have a bar chart with one number (say 700). I know that you can get an average or trend line that starts halfway through the bar and goes past it, and I know that you can do one thats wider than the bar, but how would you get an average (or trend) line of 500 fully across one bar that goes to 700 in a chart?

Hello
The way I often use is to create a second series as an XY scatter on the Secondary Axis and set its horizontal bounds to 0 and 1, then hide its axis. see attached.

Hope this helps.
DBY