Last evening I wrote a small package containing a new
stat_poly_eq() based on my previous R tips 1: Playing with polynomials.
It is simple enough that it may serve as an example of what is need under
ggplot2 >= 2.0.0 for defining a simple
stat_. I used the source code of
ggplot2 itself as a template, maybe you could just use mine given below. For each stat one needs to define one function, which will be public and an object holding all the definitions needed for the actual computations and mapping of aesthetics (the object is used internally although still needs to be exported if included in a package).
A package that as of today contains the same
stat_poly_eq() definition is available at Bitbucket. In the future I may add more examples or improve this one. It can be installed from source using package