If you are trying to find the best polynomial of degree 2 you need 3 points - as you can not fit a quadratic with only 2 points. I tested it with multiple point

If you are trying to find the best polynomial of degree 2 you need 3 points - as you can not fit a quadratic with only 2 points. I tested it with multiple points. 400,000 to 800,000, and 400,000 to 600,000. I should have done it the first time though. It does look linear but that's the problems with eye ball testing. I think if you fit a quadratic, you'll start to see the gradual curve. Its will be a very zoomed in area in a parabola. If I had more time I'd generate the dataset from the graph and try to fit linear and quadratic regression line and compare the models using coefficient of determination. That would provide some strong for/against evidence.