Removing legend box (border) in pgfplots

I'm attempting to remove or 'turn off' the legend border in pgfplots, but I can't seem to figure out how to do it. I assume it will look something like this:

\begin{axis}[ legend style={?}]


where ? is a property related to the legend border (or box) that I can't seem to find. The pgfplots help is really great, but I haven't seen this addressed anywhere. Does anyone know how to accomplish this simple task?

\begin{axis}[ legend style={draw=none}]