Good question! I assume you’re referring to the big light purple area on the graph. The reason it is U-shaped is because it is trying to fit a polynomial to your data. Since your data points start out with a big drop and then level out a bit more, the best polynomial fit is some sort of parabola that starts going down steeply, then levels out, then goes back up. Honestly I’m not sure how much sense fitting a polynomial makes for weight loss. If I were you I would just ignore it; it has no real effect on anything, it’s just a way to try to help you visualize the general trend. You can turn it off if you go into the “settings” tab for your goal, scroll down to the bottom, and uncheck “Turquoise swath”.