I managed to connect my Water Softener to HA. It also provides a list of values of the daily water usage for the last 30 days. Now I receive 30 values through 30 sensors and would like to show these in a Mini Graph Card. Is there a way to “transform” there 30 single values representing the useage of the last 30 days into a sensor that can be shown in Mini Graph Card?
You don’t need the 30 values.
I don’t know how the 30 sensors are named, but if you have something like "sensor.current_day’, “sensor.day-1”, “sensor.day-2”, …, just use sensor.current_day in the graph. The history will build itself inside HA.