@Lorchigator I actually just made a userscript for this exact use case! It is called WaniKani Review Analytics and it adds a panel to your dashboard with daily accuracy trends, weekly charts, SRS breakdown, and item type analysis.
It is exactly what you described as something that displays this for the reviews I did over a day or a week or month.
Install: WaniKani Review Analytics
Forum: [Userscript] WaniKani Review Analytics — Track your accuracy over time with charts
GitHub: bimapopo345 (Bima Prawang Saputra) · GitHub