draw x tick labels on the top layer

Question

For a group of plots i want to place all x tick labels on the top layer so that lines, that have been drawn at the end of the tikzpicture environment, are on a layer below the labels.

I already tried to set the layer with on top layer but unfortunately a scope environment seems to be always needed to draw on a specific layer.

My example below illustrates the problem. The goal is that the x tick label node at x=2 is eventually placed on top of the dashed line.

---

Example

\documentclass{standalone}

\usepackage{pgfplots}
\usepackage{tikz-layers}
\usepgfplotslibrary{groupplots}
\usetikzlibrary{backgrounds}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}[remember picture]
    \begin{groupplot}[
        no markers,
        xmax=6,
        ymin=0,
        every x tick label/.style={
            fill=red
        },
        group style = {
            group size = 1 by 2,
        },
    ]

        \nextgroupplot[ymax=15]
            \addplot +[domain=0:6] {0.5*x^2};
            \coordinate (t1g1-top) at (axis cs:2,15);
            \coordinate (t1g1-bottom) at (axis cs:2,0);

        \nextgroupplot
            \addplot  +[domain=0:6] {x};
            \coordinate (t1g2-bottom) at (axis cs:2,0);

    \end{groupplot}


    \begin{scope}[on above layer]
        \draw[dashed, ultra thick] (t1g1-top) -- (t1g2-bottom);
    \end{scope}

\end{tikzpicture}

\end{document}

Answer 1

The simplest way to do it with only a minor side effect is to use the set layers option in combination with axis on top to achieve what you want. Both options should be added to the axis options.

% used PGFPlots v1.16
\documentclass[border=5pt]{standalone}
\usepackage{pgfplots}
    \usepgfplotslibrary{groupplots}
\begin{document}
\begin{tikzpicture}
    \begin{groupplot}[
        % ---------------------------------------------------------------------
        % activate layers
        set layers,
        % and draw axis lines and labels on "top layers"
        axis on top,
        % ---------------------------------------------------------------------
        no markers,
        xmax=6,
        ymin=0,
        every x tick label/.style={
            fill=red
        },
        group style = {
            group size = 1 by 2,
        },
    ]
    \nextgroupplot[ymax=15]
        \addplot+ [domain=0:6] {0.5*x^2};
        \coordinate (t1g1-top) at (axis cs:2,15);
    \nextgroupplot
        \addplot+ [domain=0:6] {x};
        \coordinate (t1g2-bottom) at (axis cs:2,0);
    \end{groupplot}

        \draw[dashed, ultra thick] (t1g1-top) -- (t1g2-bottom);
\end{tikzpicture}
\end{document}