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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.
LaTeX can also let you create flow charts and all other sorts of diagrams through the TikZ package.
Emacs will also let you do this through its org-mode package/feature. Very useful for all kinds of organzation such as notes, todo lists, agendas, code snippets, spreadsheets. It can also export to all kinds of formats. You can see how to set up flow charts here and see how to use it here. Emacs' org mode should also support PlantUML through Babel, which is what makes ditaa possible as well.
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.