I still thinks this is "missusing" the current system: fun ends up in `self.mnaual_aes` and it is not a "aestetic mapping" (i.e. the ggplot2 doc states that the only valid aes is 'y' ->  http://docs.ggplot2.org/current/stat_function.html)... 

But I think this would anyway benefit from a different design, specifying mappings (color=<column>), settings (color="red") and "other" (fun, args,...). I open a bugreport about this...