You can find this battery usage summary in the settings.
Depending on your Android version you would have to use the three dot menu top right of the screen in Settings - Battery, or Battery usage may be an item there already.
That is really good ! My battery usage has improved as well over the version but I still have a small slanted slope during the night though it is maybe because of shelter housing whatsapp: I couldn’t tell
The statistics reset with each complete charge of the battery. Afterwards it takes a little time for the statictics to show again, as they must first be collected again.
I charged my Fairphone 2 to 100% about 1.5 hours ago, it’s now on 98% and still shows the same message you describe, so …