Placing extra x tick on the upper axis

Question

Please consider the following MWE. In addition to the xstick of the bottom x-axis (or primary x-axis) I would like to add a extra tick at the upper x-axis (or secondary x-axis).

\documentclass{article}

\usepackage{tikz}
\usepackage{pgfplots,pgfplotstable}
\usetikzlibrary{pgfplots,groupplots}

\begin{document}


\begin{tikzpicture}
    \begin{groupplot}[
        group style={
            group size=2 by 2,
            xlabels at=edge bottom,
            ylabels at=edge left,
    vertical sep= 1.5cm,
    horizontal sep= 2cm,
    every axis y label/.style={at={(ticklabel cs:0.5)},rotate=90,anchor=near ticklabel}
        },
        footnotesize,
        xlabel=xlabel,
        ylabel=very long label
        ,xtick={42,44,46,48}
        ,extra x ticks={41}
        ,extra x tick style={pos=right, grid=major,
        tick label style={pos=left,font=\footnotesize,anchor=north}},
    extra x tick labels={VLP},
]
    \nextgroupplot[title=\#1]
        \addplot[dashed] coordinates{(42, 255) (43, 1584) (44, 1296) (45, 432) (46, 972) (47, 540) (48, 1104) (49, 0)};
        \addplot[blue] coordinates{(41, 608) (42, 608) (43, 1068) (44, 1068) (45, 870) (46, 654) (47,654) (48, 654) (49, 0)};
    \nextgroupplot[title=\#2]
        \addplot[dashed] coordinates{(42, 400) (43, 400) (44, 0) (45, 400) (46, 0) (47, 0) (48, 0) (49, 0)};
        \addplot[blue] coordinates{(41, 267) (42, 267) (43, 267) (44, 200) (45, 200) (46, 0) (47, 0) (48, 0) (49, 0)};
    \nextgroupplot[title=\#3]
        \addplot[dashed] coordinates{(42, 1800) (43, 2100) (44, 1800) (45, 900) (46, 2100) (47, 2100) (48, 2100) (49, 0)};
        \addplot[blue] coordinates{(41, 1613) (42, 1613) (43, 1613) (44, 1613) (45, 1613) (46, 1613) (47, 1613) (48,1613) (49, 0)};
    \nextgroupplot[title=\#4]
        \addplot[dashed] coordinates{(42, 6800) (43, 2800) (44, 2800) (45, 2800) (46, 2000) (47, 2800) (48, 0) (49, 0)};
        \addplot[blue] coordinates{(41, 3600) (42, 3600) (43, 2799) (44, 2789) (45, 2708) (46, 2254) (47, 2254) (48, 0) (49, 0)};
    \end{groupplot}
\end{tikzpicture}


\end{document}  

Answer 1

I don't know how your document could work with that strange preamble (\usetikzlibrary{pgfplots}?). Anyway, you have to set both tick pos and ticklabel pos to right, then there is no need to set the anchor.

\documentclass{article} 

\usepackage{pgfplots} 
\usepgfplotslibrary{groupplots} 
\pgfplotsset{compat=1.6, /pgf/number format/.cd,
             1000 sep={\,}, min exponent for 1000 sep=4}

\begin{document} 


\begin{tikzpicture} 
    \begin{groupplot}[ 
        group style={ 
            group size=2 by 2, 
            xlabels at=edge bottom, 
            ylabels at=edge left, 
            vertical sep=1.5cm, 
            horizontal sep=2cm, 
            every axis ylabel/.append style={at={(ticklabel cs:0.5)}, rotate=90,
                                             anchor=near ticklabel}}, 
        width=0.46\textwidth, 
        xlabel={xlabel}, 
        ylabel={very long label}, 
        xtick={42,44,46,48}, 
        extra x ticks={41},
        extra x tick labels={VLP}, 
        extra x tick style={tick pos=right, grid=major, ticklabel pos=right,
                            tick label style={font=\footnotesize}}     
] 
    \nextgroupplot[title=\#1] 
        \addplot[dashed] coordinates{(42, 255) (43, 1584) (44, 1296) (45, 432)
                                     (46, 972) (47, 540) (48, 1104) (49, 0)}; 
        \addplot[blue] coordinates{(41, 608) (42, 608) (43, 1068) (44, 1068)
                                   (45, 870) (46, 654) (47,654) (48, 654) (49, 0)}; 
    \nextgroupplot[title=\#2] 
        \addplot[dashed] coordinates{(42, 400) (43, 400) (44, 0) (45, 400)
                                     (46, 0) (47, 0) (48, 0) (49, 0)}; 
        \addplot[blue] coordinates{(41, 267) (42, 267) (43, 267) (44, 200)
                                   (45, 200) (46, 0) (47, 0) (48, 0) (49, 0)}; 
    \nextgroupplot[title=\#3] 
        \addplot[dashed] coordinates{(42, 1800) (43, 2100) (44, 1800) (45, 900)
                                     (46, 2100) (47, 2100) (48, 2100) (49, 0)}; 
        \addplot[blue] coordinates{(41, 1613) (42, 1613) (43, 1613) (44, 1613)
                            (45, 1613) (46, 1613) (47, 1613) (48,1613) (49, 0)}; 
    \nextgroupplot[title=\#4] 
        \addplot[dashed] coordinates{(42, 6800) (43, 2800) (44, 2800) (45, 2800)
                                     (46, 2000) (47, 2800) (48, 0) (49, 0)}; 
        \addplot[blue] coordinates{(41, 3600) (42, 3600) (43, 2799) (44, 2789)
                              (45, 2708) (46, 2254) (47, 2254) (48, 0) (49, 0)}; 
    \end{groupplot} 
\end{tikzpicture} 
\end{document}