PGFPlotsX has documentation on how to interface with pgfplots, while the documentation of the latter should be used for actual plotting constructs. On a TeXLive installation,
Don’t to that. It works, but PGFPlotsX will treat "legend pos={south east}" as an option with that name (and no value). It is better to do what @fredrikekre suggested.
BTW, I made a PR to the documentation, suggestions welcome:
What about the trick with legend_pos getting converted to legend pos? That is non-obvious and except for that example there is no mention in the manual yet.