Ok got it! I believe I answered part of your question in another forum post:
System Power Penalty and Link Margin
There is also a tutorial example on calculating the receiver sensitivity:
Receiver sensitivity—Minimum input power (Optical System)
There is not a variable that is automatically created by OptiSystem that describes the Link Power budget. You could create one by using the Optical Subtractor component and then using an Optical Power Meter. Then using the result of the power meter as the variable on the y axis of your graph. Are you sweeping the transmitted power and/or the receiver sensitivity?