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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.
Qt meets all your requirements; everything (GUI code and business logic) can be implemented in moderate C++ (C++ 98 or higher, even "C with classes" style is ok). RAM usage is generally low (< 10 MB for most of my apps). If you prefer C over C++ https://nappgui.com is yet another good library.
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