Hello asif,
Here i would rather take up to the theoretical aspect of eye penalty and infer information from the graph itself.
Now as you mentioned that x-axis is the length of the link and y-axis the penalty , now what i infer from the graph is that with length of the link increasing , more losses are associated with transmission because the length has increased here and to compensate those losses , the requirement of the parameter (say power) increases linearly and then suddenly the requirement drops. Now since you are implementing the system you might be knowing the reason here. Please do explain it if you resolve it.
Regards