You have not shown what you actually did, only some apparently screwy results. You have not provided the actual data you started with, only what may be a picture of it. Is there a way to offer good help? It looks like you tried to fit a quadratic polynomial to the data in the first figure. Of course that would produce crap. Would you expect otherwise? Such a quadratic polynomial could not possibly look any different.
So slow down. State what you actually need to do. Provide some data, not just pictures of data. Tell us what model you want to use.
If you append some data, then do so, attached to a comment on your question, or by editing your question, not as an answer.
