The answer to this question uses a PANEL variable which seems to be internal to ggplot. But searching the ggplot documentation and also Hadley Wickham's book, I can find no reference to it at all. Is this documented anywhere?
Also, looking at the code for stat_bin(...), there is evidently a vector count created (which contains the count of y for each unique x??). This is also accessible in aes(...) but, again, I can find no documentation.
So my question is: is there a place where all of these internal variables are documented, or must one just go to the code?