It is quite hard to find historic data for German cities. Usually you can only look up the historic data on the city's own web pages. However, each page is different, so you have to search on each page where you can actually find the information (if it is even present).
The aim of this project is to provide easy access to this historic data.
You can find the graph of the past week here: https://klassm.github.io/covid-historie/