Difference with Sigma.js

Hey, Andrei!
What is the key difference between ngraph and [Sigma.js](https://github.com/jacomyal/sigma.js) libs? I need to choose a library to display dynamic server-side updated graph. Is there some memory or performance issues to look on?

The issue names no files, tests, or entry points. Start by reviewing the ngraph and Sigma.js project documentation and defining the graph-update, memory, and performance criteria; the work is done only when those differences and a library recommendation are documented.

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