Is there documentation for which functions work as
pgfplots expressions (like sin, cos, exp, etc.) The
pgfplots documentation doesn't seem to say anything on what you can/can't use.
pgfplots package uses
pgfmath expressions from the
pgf package. See the TikZ/PGF manual (
pgfmanual.pdf) in part VI, section 63 "Syntax for mathematical expressions", page 527 in PGF v2.10 for a list of all available PGF math expressions. You can also add your own, but some programming is required for that.