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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.
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libui
Simple and portable (but not inflexible) GUI library in C that uses the native GUI technologies of each platform it supports.
Fl_Flex (shameless plug!) is now officially in upstream FLTK since 1.4 for a "flexbox style" layout manager.
Fl_Flow (another shameless plug!) is available for a slightly more novel layout system. This one I feel will work better with C if wrapped.
What about https://github.com/andlabs/libui
I think good option would be nuklear it is a single header lib
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