use the coordinates of an axis tip with pgf plot

Question

here's a drawing I just made (the sine function are replaced by external datas)

\documentclass[12pt]{article}
\usepackage{pgfplots}
\usepackage{tikz}
\usetikzlibrary{through,calc,arrows,decorations.pathmorphing,backgrounds,positioning,fit}


\begin{document}
\begin{figure}
 \begin{tikzpicture}
 \def\decalx{-0.8};
 \def\decaly{-0.9};


 \foreach \pas in {340,320,...,0} 
{
\begin{scope}[xshift=\decalx*(340-\pas),yshift=\decaly*(340-\pas)]
    \begin{axis}[xmax=600,ymin=-1,ymax=1,xtick=\empty,ytick=\empty,axis x line=middle,axis y line = left]
       \addplot[fill=black!5!white,fill opacity=0.5, domain=0:360] expression {sin(\pas/100*\x)};
    \end{axis}
    \end{scope}
    }

\begin{scope}[xshift=-2.8cm,yshift=-10.8cm]
\begin{axis}[,x={(-\decalx,-\decaly)},anchor=south west,xmin=0,xticklabel style={xshift=5},xtick={90,270},axis x line=middle,hide y axis]
\addplot[blue,samples=19,domain=0:360,error bars/.cd, y dir=minus,y fixed relative=1, error mark=none, error bar style={dotted}]  {sin(x)};
\end{axis}
\end{scope}
\end{tikzpicture}
\end{figure}

\end{document}

The thing is the blue curve is placed by trial and error, depending on the width, height, y min and max values and a bunch of other stuffs... Is there any way to extract the coordinate of the foreground plot's x axis so that I can draw the final blue chart from it ??? the hard part is probably that this blue curved is skewed...

I've been doing this by hand so far and I can definitely keep on doing it, it's just that I have to make tens of these, and it takes forever to compile with my 300 points data tables...

thanks !!!!

//EDIT///

If I need a ymin or ymax parameter Ant's solution doesn't work anymore...

\documentclass[12pt]{article}
\usepackage{pgfplots}
\usepackage{tikz}
\usetikzlibrary{through,calc,arrows,decorations.pathmorphing,backgrounds,positioning,fit}


\begin{document}
\begin{figure}
 \begin{tikzpicture}
 \def\decalx{-0.6};
 \def\decaly{-0.9};
\def\ymin{-0.5};

 \foreach \pas in {340,320,...,0} 
{
\begin{scope}[xshift=\decalx*(340-\pas),yshift=\decaly*(340-\pas)]
    \begin{axis}[xmax=600,ymin=\ymin,ymax=1,xtick=\empty,ytick=\empty,axis x line=middle,axis y line = left]
       \addplot[fill=black!5!white,fill opacity=0.5, domain=0:360] expression {sin(\pas/100*\x)};
    \end{axis}
    \end{scope}
    }


    \begin{scope}[xshift=\decalx*(340),yshift=\decaly*(340)]
    \begin{axis}[xmax=600,ymin=\ymin,ymax=1,xtick=\empty,ytick=\empty,axis x line=middle,axis y line = left]
       \addplot[fill=black!5!white,fill opacity=0.5, domain=0:360]
       expression {0};
       \node (start blue axis here) at (rel axis cs:1, 0) {};
     \end{axis}
  \end{scope}


  \begin{axis}[at=(start blue axis here), x={(-\decalx,-\decaly)},anchor=south
    west,xmin=0,xticklabel style={xshift=5},xtick={90,270},axis x
    line=middle,hide y axis]
    \addplot[blue,samples=19,domain=0:360,error bars/.cd, y
    dir=minus,y fixed relative=1, error mark=none, error bar
    style={dotted}] {sin(x)};
\end{axis}

\end{tikzpicture}
\end{figure}

\end{document}

Answer 1

You can place a node, when drawing the last rectilinear axis, to which you align the skew axis. See below for an annotated version of your example:

\documentclass[12pt]{article}
\usepackage{pgfplots}
\usepackage{tikz}
\usetikzlibrary{through,calc,arrows,decorations.pathmorphing,backgrounds,positioning,fit}


\begin{document}
\begin{figure}
 \begin{tikzpicture}
 \def\decalx{-0.8};
 \def\decaly{-0.9};


 \foreach \pas in {340,320,...,20} 
{
\begin{scope}[xshift=\decalx*(340-\pas),yshift=\decaly*(340-\pas)]
    \begin{axis}[xmax=600,ymin=-1,ymax=1,xtick=\empty,ytick=\empty,axis x line=middle,axis y line = left]
       \addplot[fill=black!5!white,fill opacity=0.5, domain=0:360] expression {sin(\pas/100*\x)};
    \end{axis}
    \end{scope}
    }

All of this is as before, except that I've taken the last axis out of the foreach loop and added it below.

\begin{scope}[xshift=\decalx*(340),yshift=\decaly*(340)]
    \begin{axis}[xmax=600,ymin=-1,ymax=1,xtick=\empty,ytick=\empty,axis x line=middle,axis y line = left]
       \addplot[fill=black!5!white,fill opacity=0.5, domain=0:360]
       expression {0};
       \node (start blue axis here) at (axis cs:\pgfkeysvalueof{/pgfplots/xmax}, 0) {};
     \end{axis}
  \end{scope}

The rel axis cs coordinate system makes it easy to put the node where the skew axis should start. And now we just use the at key in place of the scope you had before:

  \begin{axis}[at=(start blue axis here), x={(-\decalx,-\decaly)},
    anchor=left of origin,
    xmin=0,xticklabel style={xshift=5},xtick={90,270},axis x
    line=middle,hide y axis]
    \addplot[blue,samples=19,domain=0:360,error bars/.cd, y
    dir=minus,y fixed relative=1, error mark=none, error bar
    style={dotted}] {sin(x)};
\end{axis}

\end{tikzpicture}
\end{figure}

\end{document}

...which produces the same diagram as the original example.

Edited to align by x axes rather than bottom right corner, so that you can change the y axis scale, following discussion in comments.