Good question! Mike wrote a notebook about getting data into Observable here: https://beta.observablehq.com/@mbostock/introduction-to-data - the gist is that right now any host that supports CORS does the trick, and the combination of GitHub Gist & RawGit is pretty good for the task. It'd be nice to drag & drop data right in the app, though, so... stay tuned.

And, yep - something like pandas for JavaScript would also be nice... also stay tuned.