legend placement with bounding box restrictions and externalization in pgfplots

Question

I find it useful to add

\pgfplotsset{/tikz/every picture/.append style={trim axis left,trim axis right}}

to my preamble for bounding box restrictions. But then when I use the legend to name feature (see attachment) I get the following error:

! Package pgf Error: No shape named current axis is known

How can I fix this?

MWE:

\documentclass{article}

\usepackage{lipsum,pgfplots}
   \pgfplotsset{compat=1.7}

\usepgfplotslibrary{external}
\usetikzlibrary{pgfplots.external}

   \tikzexternalize[shell escape=-enable-write18]
   \pgfplotsset{/tikz/every picture/.append style={trim axis left,trim axis right}}

\begin{document}

\setlength\fboxsep{0pt}

\begin{figure}[h!]\centering

\fbox{\begin{tikzpicture}
\begin{axis}[domain=-2:2,
             width=0.5\textwidth,
             xlabel={$x$},ylabel={$y$},
             %
             legend to name=mylegend,
             legend columns=-1,
             legend entries={$x^2$,$x^3$}]
\addplot {x^2};\addplot {x^3};
\end{axis}
\end{tikzpicture}}%
%
\hspace{1.5cm}%
%
\fbox{\begin{tikzpicture}
\begin{axis}[domain=-2:2,
             width=0.5\textwidth,
             xlabel={$x$},ylabel={$y$}]
\addplot {x^2};\addplot {x^3};
\end{axis}
\end{tikzpicture}}%

\tikzexternaldisable\ref{mylegend}\tikzexternalenable
                    % http://tex.stackexchange.com/q/37471/14497
\end{figure}

Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus elit,
vestibulum ut, placerat ac, adipiscing vitae, felis.

\end{document}

Answer 1

It turns out one susprisingly simple solution is to redefine the style every picture with

\pgfplotsset{/tikz/every picture/.style={}}

before calling \ref. With regard to my MWE:

{\pgfplotsset{/tikz/every picture/.style={}}
\tikzexternaldisable\ref{mylegend}\tikzexternalenable
}

does the trick.