My take on this is the following: d3 / js are great because they provide a portable expressive substrate for providing rich interactive visuals. At the same ti

My take on this is the following: d3 / js are great because they provide a portable expressive substrate for providing rich interactive visuals. At the same time, If I'm writing a tool to do data vis (say in R, Haskell, Scale, etc), while I may want to leverage browser based rendering, I really really really have no interest in "compiling" my visuals to javascript code. My take on vega is it provides a nice "shim" so I can emit a visual which will be rendered to d3, without writing a mini javascript compiler. tl;dr Vega (as they say in their documentation on the github wiki), is a data format layer that makes it easier for high level data vis tools to emit a format which is used to construct the D3 rendering.