Could anyone explain the benefit of using Dc.js vs google charts API or d3.js? Crossfilter seems really cool - but since it's another library, what is it that Dc is offering?
dc.js marries crossfilter.js with d3.js -- that's it in a nutshell.
crossfilter allows you to group and filter your data in-memory very quickly, and potentially for a LOT of data See http://bl.ocks.org/milroc/7032589 for a good intro to crossfilter (launch the demo on slide #39 -- loads 36M flight records).
you likely already know what d3 does. dc.js makes it easy to generate dashboards where the user can interact with the charts (e.g. filter by Q1, Q2...see the NASDAQ example at https://dc-js.github.io/dc.js/)