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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.
There's also a pull request on the new pdf-tools that allows one to use the keyboard to select text as well. I don't think it has been added to the project yet and when I pasted the code into my init.el, it didn't work well (highlight not on text), but it is apparently a milestone so hopefully something will come of it.
Also if you're annotating plain text, org-remark is great. I love not having to use the mouse to select text.
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