First, do you really really want to write a Skeinforge plugin ?
Preliminary notes: some sites such as wired link to this post, but you may like to read my less technical review about temperature gradients applied to wood first. Also, you could check this post if you just want to try it.
Now, read on if you are a programmer and want to write your own plugin for Skeinforge. Note also that Cura no more uses Skeinforge as it has a much easier plugin system (that cannot do as much as what can be done with Skeinforge though).
Indeed and contrary to what was being done so far, I did not want to manually insert the M104 temperatures changes in the file sent to the printer, nor stay close to the ulticontroller. So I designed a plugin for the Cura / Skeinforge software pair.
Since that was not trivial, I document the procedure here.