Since the line may not have a constant value, I created a simple example where the target changes. I have attached that file. I can create a chart that looks like this:
In Graph Builder, drag Day to the X-axis. Drag Daily Sales and Sales Target to the Y-axis. Change the graphing element to the bar chart. Change the Bar Style to Single.
Another option is to go with a more continuous line for the Sales Target.
You can get that as well, but has a few more steps.
Drag Day to the X-axis. Drag Daily Sales and Sales Target to the Y-axis. Change the graphing element to the Bar Chart. Click the Variables triangle and clear the checkmark from the Sales Target.
Hold the Shift key down and click the Line graphing element. Under the Line properties, click the Variables triangle and uncheck the Daily Sales variable.
Even if neither of these are what you want, I think the steps will show you some different ways to work with Graph Builder to get the picture that you want.
Dan Obermiller