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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.
Again there are many great free ones out there, but my personal recommendation is definitely this one. It is paid, but the cost is not far off a pint of beer these days and you can customise it however you want (you could think of it as an investment in terms of the value it brings into your life vs the cost).
NOTE:
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Hence, a higher number means a more popular project.