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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.
I guess I'm always surprised when I don't see MapTool mentioned. It's multiplatform, 100% open source and the Takehara framework includes just about all of 5e. No doubt there is some learning curve but you can do everything with it that any other platform does. https://www.rptools.net/toolbox/maptool/. https://github.com/rtakehara/5e-Framework/
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.