Porting D3.js to other languages
d3.js, svg, visualization
Solution
As some people have commented, beyond D3 relying on some of the features of Javascript (not just syntactically, but even more so with respect to the nature of the language itself) - it also depends on the DOM and some means of interpreting SVG markup -- all of which is included in a browser environment.
Have you considered the R programming language? I've always been under the impression that one of the voids D3 fills is providing some of the same data-driven approach to visualization that is supported in R, but just in a browser environment.
Now - that's not to say that R can do everything that D3 can do or vice versa - but there is a lot of overlap as far as visualization is concerned and depending on the nature of your needs - R might be just what you're looking for. And if that's the case, there is a great library called RInside and another called rcpp that aim to provide integration of C++ in R and R in C++.
Problem
I am looking to use D3.js in a c++ application, where using js isn't feasible for a variety of reasons. What elements of js/svg/css are being relied on in the code that would not be easily substituted with svg/vector type drawing functions from a c++ library. Where am I likely to run into difficulty?