You can also find some Reddit posts on how to restore the graph. It's probably a system bug that came with the latest One UI 3.0 update. Hopefully there'll be a systems patch/update to fix it. Hope it helps :)
Here’s what you can do: 1. Open the Settings app on the phone, then go into the Apps submenu. 2. Here, tap the small icon next to the Your apps text, then enable the Show system apps option and press OK. 3. Now, scroll down until you find the Samsung Device Health Manager Service app in the list and tap it. 4. Here, go into the Storage submenu, and hit the Clear data option at the bottom. 5. Fully charge your phone. You will see the battery stats again after a few minutes.