[Tex/LaTex] How to change x-axis label position only horizontally keeping the default vertical value using pgfplots

pgfplots

I have two plots of different width next to each other. I want them to share one x-axis label. For this I take the x-axis label of the left plot and shift it to the right so that it is perfectly centred below the combined graph. I do this using 

x label style={at={(axis description cs:1./2./0.7/0.95,-0.08)},anchor=north}

However, now the label has a different distance to the x-axis tick labels than in all other plots due to the -0.08. Is there any way I can get a default value so that I do not have to approximate the default by some made up value?

I found the following SE question that is somewhat related: Default distance between axis numbers and axis labels pgfplots. It seems 2*inner sep + 2*outer sep = 2*0.3333em + 2*0.5\pgflinewidth is the default distance. But how do I translate this into the axis description cs?

If there is a more elegant way of doing all of this please mention it. I am not bound to use the above command.

Edit 1: Sample code

\documentclass[]{scrartcl}

\usepackage{pgfplots,pgfplotstable,tikz}
\pgfplotsset{compat=1.9}

\usepackage[utf8]{inputenc}
\usepackage{siunitx}



\begin{document}


\begin{figure}
\centering
\begin{tikzpicture}
    \pgfplotsset{
        myaxis/.style={
            width=0.8\textwidth*0.7*0.95,
            height=0.8\textwidth/1.61803398875,
        }
    }
    %
    \begin{semilogyaxis}[
        myaxis,
        xlabel=This should be centered,
        ylabel=Y-Label,
        ymin=0.01,
        ymax=1e11,
        xmin=-0.5,
        xmax=6.5,
        scale only axis,
        x label style={at={(axis description cs:1./2./0.7/0.95,-0.08)},anchor=north},
    ] %
    \draw
        (axis cs:3.5,0.13)coordinate(ul)--
        (axis cs:6.5,0.13)coordinate(ur)--
        (axis cs:6.5,1.6)coordinate(or)--
        (axis cs:3.5,1.6) -- cycle;
    \end{semilogyaxis} %
    \begin{axis}[
        myaxis,
        xshift=0.8\textwidth*0.7+0.8\textwidth*0.3*0.05,
        width=0.8\textwidth*0.3*0.95,
        xmin=3.5,
        minor y tick num = 4,
        xmax=6.5,
        ymin=0.13,
        ymax=1.6,
        restrict y to domain=0.13:1.6,
        restrict x to domain=3.5:6.5,
        %trim axis left,
        yticklabel pos=right,
        scale only axis,
        ]
    \end{axis}
    \draw
        (current axis.north west)--(or)
        (current axis.south west)--(ur);
\end{tikzpicture} %
\end{figure}

\end{document}

Best Answer

Different from Harish Kumar's solution, this solution uses xticklabel cs coordinate to place an extra node without using x label style macro with xshift command.

enter image description here

Code

\documentclass[]{scrartcl}

\usepackage{pgfplots,pgfplotstable,tikz}
\pgfplotsset{compat=1.9}

\usepackage[utf8]{inputenc}
\usepackage{siunitx}

\begin{document}

\begin{figure}
\centering
\begin{tikzpicture}
    \pgfplotsset{
        myaxis/.style={
            width=0.8\textwidth*0.7*0.95,
            height=0.8\textwidth/1.61803398875,
        }
    }
    %
    \begin{semilogyaxis}[
        myaxis,
        xlabel=This should be centered (Harish Kumar's),
        ylabel=Y-Label,
        ymin=0.01,
        ymax=1e11,
        xmin=-0.5,
        xmax=6.5,
        scale only axis, clip=false,
        x label style={at={(axis description cs:1./2./0.7/0.95,-0.08)},anchor=north},
        x label style={xshift=0.8\textwidth*0.5-0.8\textwidth*0.7*0.95*0.5}
    ] %  Harish Kumar's solution
    \draw
        (axis cs:3.5,0.13)coordinate(ul)--
        (axis cs:6.5,0.13)coordinate(ur)--
        (axis cs:6.5,1.6)coordinate(or)--
        (axis cs:3.5,1.6) -- cycle;
        \node[red] () at (xticklabel cs: 1,25pt) {This should be centered (Proposed)}; 
        % adjust 20pt for variants
    \end{semilogyaxis} %
    \begin{axis}[
        myaxis,
        xshift=0.8\textwidth*0.7+0.8\textwidth*0.3*0.05,
        width=0.8\textwidth*0.3*0.95,
        xmin=3.5,
        minor y tick num = 4,
        xmax=6.5,
        ymin=0.13,
        ymax=1.6,
        restrict y to domain=0.13:1.6,
        restrict x to domain=3.5:6.5,
        %trim axis left,
        yticklabel pos=right,
        scale only axis,
        ]
    \end{axis}
    \draw
        (current axis.north west)--(or)
        (current axis.south west)--(ur);

\end{tikzpicture} %
\end{figure}

\end{document}
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