You are right: the graph does not represent a proportion, because it does not go thru the origin.
If this is correct the answer would be
y^2 + 0.1y + 0.0025
If they get unexpected results they could note what they could've done wrong or what they could change in a next trail of experiments. Or look back and see what different happened to they could've hypothesized. There could be many different courses on what to do next.