Have you tried adding the goal line as an extra column in your dataset, and then adding it to the graph as a line series?
I'm assuming your dataset looks like this, with the addition of a Goal column:
Assumed Returns Linear Returns Goal
--------------- -------------- ---------
0.0 45,243 40,000
0.5 48,869 40,000
Add a new series to the chart based on the Goal column and then change it's series type to line.