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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
The app is free, open source, and on the Amazon App store where they made me rename so people didn't think it was a bitcoin miner (Google Play review pending for 3 years, I doubt it will happen any time soon). You can also sideload from the releases section in github. In theory this works on iOS as well, but I don't want to blow a $100/year fee for this- if you have iOS development set up, feel free to build the app yourself. It should work (in theory).
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.