How to only show legend in pgf plot?

I have created with matlab2tikz a standalone picture, saved in myfile.tikz

Here is the code:

  % This file was created by matlab2tikz v0.5.0 running on MATLAB 8.4.%Copyright (c) 2008--2014, Nico Schlömer <nico.schloemer@gmail.com>%All rights reserved.%Minimal pgfplots version: 1.3%%The latest updates can be retrieved from%  http://www.mathworks.com/matlabcentral/fileexchange/22022-matlab2tikz%where you can also make suggestions and rate matlab2tikz.%\documentclass[tikz]{standalone}\usepackage{pgfplots}\usepackage{grffile}\pgfplotsset{compat=newest}\usetikzlibrary{plotmarks}\usepackage{amsmath}\begin{document}\begin{tikzpicture}\begin{axis}[%width=5.828221in,height=4.83871in,  y tick label style={        /pgf/number format/.cd,            fixed,            precision=2,        /tikz/.cd    },at={(0in,0in)},xlabel={Amount of trajectories $(\times 10^7 )$},ylabel={4th largest eigenvalue},scale only axis,separate axis lines,every outer x axis line/.append style={white!15!black},every x tick label/.append style={font=\color{white!15!black}},xmin=10,xmax=50,every outer y axis line/.append style={white!15!black},every y tick label/.append style={font=\color{white!15!black}},ymin=0,ymax=0.4,legend style={draw=white!15!black,legend cell align=left}]\addplot [color=red,solid,mark=square*,mark options={solid,fill=red}]  table[row sep=crcr]{%10  0.263363834999385\\11  0.267956137706241\\12  0.234749040704818\\13  0.179095397783884\\14  0.15598488764853\\15  0.107130784874058\\16  0.102335812851717\\17  0.15130873420001\\18  0.13294322038256\\19  0.0960614078900675\\20  0.0668875403805097\\21  0.0696202544023205\\22  0.0769402122493772\\23  0.0814760237222062\\24  0.0697776185374109\\25  0.0890537892164226\\26  0.0966623314755249\\27  0.0934599345653749\\28  0.0878880498733259\\29  0.087473866000113\\30  0.0892704892496597\\31  0.0812007102089455\\32  0.0785448212326198\\33  0.0772843849145571\\34  0.0748381690948553\\35  0.076911866585151\\36  0.0690916321763139\\37  0.0681618703751155\\38  0.0538871718086814\\39  0.0462813570913469\\40  0.042327952187128\\41  0.0545656572749185\\42  0.0502509655943423\\43  0.0548481613029811\\44  0.0565260239184772\\45  0.0574958892089851\\46  0.0565141460485921\\47  0.0607219533955611\\48  0.0621210992930102\\49  0.0584171708241252\\50  0.0554092144619335\\};\addlegendentry{Standart estimation};\addplot [color=blue,dash pattern=on 1pt off 3pt on 3pt off 3pt,mark=asterisk,mark options={solid}]  table[row sep=crcr]{%10  0.357550039744943\\11  0.356972039378709\\12  0.334368502350565\\13  0.30613803777209\\14  0.298223941905334\\15  0.343030324802327\\16  0.344206127225485\\17  0.342654565258385\\18  0.26909949152418\\19  0.244071723963408\\20  0.242341053266382\\21  0.242231681701872\\22  0.23431468104395\\23  0.234032984140502\\24  0.225001418677598\\25  0.187193611177314\\26  0.195346678096106\\27  0.194987732958593\\28  0.193281499543124\\29  0.193039372776092\\30  0.188745282175345\\31  0.18389991087615\\32  0.184250604858391\\33  0.18358622178072\\34  0.183425365322753\\35  0.183581339448476\\36  0.169460638908507\\37  0.166155309985801\\38  0.160320243705676\\39  0.14731116222724\\40  0.144469472179117\\41  0.143751331186272\\42  0.143538751559955\\43  0.142526804916181\\44  0.142099807792203\\45  0.142164474247563\\46  0.137436117746078\\47  0.13816942742473\\48  0.137543995531934\\49  0.137932768120272\\50  0.137592136659409\\};\addlegendentry{Corrected Frobenius estimation};\end{axis}\end{tikzpicture}%\end{document}

In my terminal, I can produce a pdf file of my picture by entering

pdflatex myfile.tikz

This is how it looks:

I want to produce a PDF file only containing the legend of the pgfplot. Is this possible?

Edit: I would like to have this outcome:

Whereas LaRiFaRi's solution provides this:

How can I get the first picture? Do I need to trial-and-error or is there a shorter version for it? Removing xmax, ymax, xmin and ymin produced an empty image.