pgfplots: horizontal distance between two plots

Question

I'm trying to plot the following graph using pgfplots:

This plot was generated with matlab.

I can easily plot the two curves with pgfplots but I have no clue how to add the vertical line with the 0.3dB marker on it. Maybe there is a possibility to extract the (interpolated) values for x (SNR) of both plots at y=10^-1. It should be mentioned that only the y values for 20, 22, 24, 26, 28, 30 and 32 dB are available.

This is my the current TeX Code (the plot data does not exactly fit to the matlab pic, but the idea is the same):

\documentclass[a4paper, ngerman]{scrreprt}
\usepackage{color}
% predefined colors
\definecolor{color1}{rgb}{0.44706,0.44706,0.44706}%
\definecolor{color2}{rgb}{0.09412,0.34902,0.66275}%
\definecolor{color3}{rgb}{0.92941,0.17647,0.18039}%
\definecolor{color4}{rgb}{0.00000,0.54902,0.27843}%
% include pgfplots 
\usepackage{pgfplots}
\usepackage{pgfplotstable}
% custom settings
\pgfplotsset{compat=1.3}
\pgfplotsset{every axis/.append style={line width=1pt}}       % line widths
\pgfplotsset{                                                 % customized size (x1.3)
  mysize/.style={
  /pgfplots/width=312pt,
  /pgfplots/height=269pt,
  /pgfplots/max space between ticks=35}
}
\pgfplotsset{                                                 % fer plot config
  ferplotcfg/.style={
    /pgfplots/xmin  = 20,
    /pgfplots/xmax  = 32,
    /pgfplots/ymin  = 10e-3, 
    /pgfplots/ymax  = 10e-1,
    /pgfplots/grid  = both,
    minor grid style = {dashed},
    major grid style = {dashed}}
}
\pgfplotsset{                                                 % reference curve cfg
  line_ref/.style={
    color1,
    /tikz/smooth,
    /tikz/mark=none,
    /tikz/style={dashed}}
}
\pgfplotsset{                                                 % reference curve cfg
  line_1/.style={
    color2,
    /tikz/smooth,
    /tikz/mark=o}
}

\begin{document}

\centering
\begin{tikzpicture}
  \begin{axis} [
    title = FER Performance,
    legend entries = {Reference, Fixed Point},
    xlabel = {SNR (dB)}, ylabel = {FER},
    ferplotcfg, mysize, ymode = log,
    ]
    \addplot[line_ref] table[x=snr, y=reference, col sep=comma, row sep=crcr] {
      snr,matlab_full_sim_fxp,reference\\
      20,0.93682,0.92946\\
      22,0.76335,0.73643\\
      24,0.39312,0.3622\\
      26,0.11584,0.10718\\
      28,0.036743,0.029785\\
      30,0.012695,0.010864\\
      32,0.0064697,0.0039063\\}; 
    \addplot[line_1] table[x=snr, y=matlab_full_sim_fxp, col sep=comma, row sep=crcr] {
      snr,matlab_full_sim_fxp,reference\\
      20,0.93682,0.92946\\
      22,0.76335,0.73643\\
      24,0.39312,0.3622\\
      26,0.11584,0.10718\\
      28,0.036743,0.029785\\
      30,0.012695,0.010864\\
      32,0.0064697,0.0039063\\}; 
  \end{axis}
\end{tikzpicture}
\end{document}

Answer 1

One way to accomplish this is to combine the techniques from pgfplots: Placing node on a specific x-position and Converting unit to coordinate works for x-coordinate, but not for y-coordinate for first placing coordinates at the specified y position and then calculating the difference:

\documentclass{standalone}
% include pgfplots 
\usepackage{pgfplots}
\usetikzlibrary{intersections, calc}
% custom settings
\pgfplotsset{compat=newest}
\pgfplotsset{                                                 % customized size (x1.3)
  mysize/.style={
  /pgfplots/width=312pt,
  /pgfplots/height=269pt,
  /pgfplots/max space between ticks=35}
}
\pgfplotsset{                                                 % fer plot config
  ferplotcfg/.style={
    /pgfplots/xmin  = 20,
    /pgfplots/xmax  = 32,
    /pgfplots/ymin  = 10e-3, 
    /pgfplots/ymax  = 10e-1,
    /pgfplots/grid  = both,
    %minor grid style = {dashed},
    %major grid style = {dashed}
    }
}
\pgfplotsset{                                                 % reference curve cfg
  line_ref/.style={
    color1,
    /tikz/smooth,
    /tikz/mark=none,
    /tikz/style={dashed}}
}
\pgfplotsset{                                                 % reference curve cfg
  line_1/.style={
    color2,
    /tikz/smooth,
    /tikz/mark=o}
}

\makeatletter
\def\parsenode[#1]#2\pgf@nil{%
    \tikzset{label node/.style={#1}}
    \def\nodetext{#2}
}

\tikzset{
    add node at y/.style 2 args={
        name path global=plot line,
        /pgfplots/execute at end plot visualization/.append={
                \begingroup
                \@ifnextchar[{\parsenode}{\parsenode[]}#2\pgf@nil
            \path [name path global = position line #1]
                ({axis cs:0,#1}-|{rel axis cs:0,0}) --
                ({axis cs:0,#1}-|{rel axis cs:1,1});
            \path [
                name intersections={
                    of={plot line and position line #1},
                    name=intersection
                },
            ] (intersection-1)
            node [label node] {\nodetext};
            \endgroup
        }
    }
}


\begin{document}

\centering
\begin{tikzpicture}
  \begin{axis} [
    ferplotcfg, mysize, ymode = log,
    ]
    \addplot[black, add node at y={0.1}{[coordinate, name=pointA]}] table[x=snr, y=reference, col sep=comma, row sep=crcr] {
      snr,matlab_full_sim_fxp,reference\\
      20,0.93682,0.92946\\
      22,0.76335,0.73643\\
      24,0.39312,0.3622\\
      26,0.11584,0.10718\\
      28,0.036743,0.029785\\
      30,0.012695,0.010864\\
      32,0.0064697,0.0039063\\}; 
    \addplot[blue, add node at y={0.1}{[coordinate, name=pointB]}] table[x=snr, y=matlab_full_sim_fxp, col sep=comma, row sep=crcr] {
      snr,matlab_full_sim_fxp,reference\\
      20,0.93682,0.92946\\
      22,0.76335,0.73643\\
      24,0.39312,0.3622\\
      26,0.11584,0.10718\\
      28,0.036743,0.029785\\
      30,0.012695,0.010864\\
      32,0.0064697,0.0039063\\};
      \draw let \p1=(pointA), \p2=(pointB) in (pointA) -- (pointB) node [pos=0, left] {%
        \pgfplotsconvertunittocoordinate{x}{\x1}%
        \pgfplotscoordmath{x}{datascaletrafo inverse to fixed}{\pgfmathresult}%
        \edef\valueA{\pgfmathresult}%
        \pgfplotsconvertunittocoordinate{x}{\x2}%
        \pgfplotscoordmath{x}{datascaletrafo inverse to fixed}{\pgfmathresult}%
        \pgfmathparse{\pgfmathresult - \valueA}%
        \pgfmathprintnumber{\pgfmathresult} dB
        };
  \end{axis}
\end{tikzpicture}
\end{document}