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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.
languages: C++..and yeah, pretty much everything else ;) Website mentions "Python, Perl, Ruby and many other languages"
FLTK
I would just add, fyne a promising newcomer, for Go.
If it comes to go, also got3 seems worth of mention. It doesn't include all GTK3 bindings, but most of them, and it's easy to use.
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.