Currently in an early stage of development, but will be a very active project. It may change quite a bit from its current state, but will always try to follow the style in which d3.js was done.
If one of [the existing models](https://github.com/novus/nvd3/tree/master/src/models) doesn't meet your needs, fork the project, implement the model and an example using it, send us a pull request, for consideration for inclusion in the project.