Add single value as constant line to diagram

I have a range A1:B of which I make a simple line diagram. I also have a single value in another cell, say C1, which I want to use as a constant reference line in the diagram.

I know I could populate the whole C column with C1 and then extend the diagram to the range A1:C. But I was wondering if there is a better way to add a value as a constant line in a diagram without wasting a whole column.

Example data

Year        Expenses    My Goal
2007        500$        500$
2008        400$
2009        550$
2010        450$
2011        300$