Two level labels in bar plot

Question

I need to group the labels in the x axis by putting a second level label. I need to separate the nodes that are grouped from the other ones or marked them somehow.

For example, if I have the labels in the example below I would like to have

S1 S2 S3  S4 S5 S6 S7   S8 S9 S10
X Group     Y Group      Z Group

How can I achieve this? I check the manual of pgfplots without any hint. Is there a special name for what I'm trying to achieve?

\documentclass{standalone}

\usepackage{tikz}
\usepackage{pgfplots}

\pgfplotstableread{
1   19.178  26.027  8.219   6.849   39.726
2   54.795  21.918  4.110   6.849   12.329
3   28.767  16.438  6.849   8.219   39.726
4   63.014  2.740   2.740   2.740   28.767
5   90.411  1.370   6.849   0.000   1.370
6   15.068  2.740   16.438  8.219   57.534
7   67.123  0.000   0.000   0.000   32.877
8   72.603  6.849   5.479   0.000   15.068
9   56.164  12.329  6.849   4.110   20.548
10  50.685  4.110   8.219   1.370   35.616
}\datatable

\begin{document}
\makeatletter
\begin{tikzpicture}
\begin{axis}[
    ylabel=label,
    xtick=data,
    xticklabels={S1,S2,S3,S4,S5,S6,S7,S8,S9,S10},
    enlarge y limits=false,
    enlarge x limits=0.1,
    ymin=0,ymax=100,
    ybar stacked,
    bar width=10pt,
    legend style={
      font=\footnotesize,
      cells={anchor=west},
      legend columns=5,
      at={(0.5,-0.15)},
      anchor=north,
      /tikz/every even column/.append style={column sep=0.2cm}
    },
]

\addplot table[x index=0,y index=1] \datatable;
\addplot table[x index=0,y index=2] \datatable;
\addplot table[x index=0,y index=3] \datatable;
\addplot table[x index=0,y index=4] \datatable;
\addplot table[x index=0,y index=5] \datatable;
\legend{Far,Near,Here,There,NotThere}
\end{axis}
\end{tikzpicture}
\end{document}

Answer 1

You could adapt the approach described in How can I mix an "ybar" and an "ybar stacked" with pgfplots? to place the group labels underneath the individual ones:

\documentclass{standalone}

\usepackage{tikz}
\usepackage{pgfplots}

\newcounter{groupcount}
\pgfplotsset{
    draw group line/.style n args={5}{
        after end axis/.append code={
            \setcounter{groupcount}{0}
            \pgfplotstableforeachcolumnelement{#1}\of\datatable\as\cell{%
                \def\temp{#2}
                \ifx\temp\cell
                    \ifnum\thegroupcount=0
                        \stepcounter{groupcount}
                        \pgfplotstablegetelem{\pgfplotstablerow}{[index]0}\of\datatable
                        \coordinate [yshift=#4] (startgroup) at (axis cs:\pgfplotsretval,0);
                    \else
                        \pgfplotstablegetelem{\pgfplotstablerow}{[index]0}\of\datatable
                        \coordinate [yshift=#4] (endgroup) at (axis cs:\pgfplotsretval,0);
                    \fi
                \else
                    \ifnum\thegroupcount=1
                        \setcounter{groupcount}{0}
                        \draw [
                            shorten >=-#5,
                            shorten <=-#5
                        ] (startgroup) -- node [anchor=north] {#3} (endgroup);
                    \fi
                \fi
            }
            \ifnum\thegroupcount=1
                        \setcounter{groupcount}{0}
                        \draw [
                            shorten >=-#5,
                            shorten <=-#5
                        ] (startgroup) -- node [anchor=north] {#3} (endgroup);
            \fi
        }
    }
}


\pgfplotstableread{
1   19.178  26.027  8.219   6.849   39.726 1
2   54.795  21.918  4.110   6.849   12.329 1
3   28.767  16.438  6.849   8.219   39.726 1
4   63.014  2.740   2.740   2.740   28.767 2
5   90.411  1.370   6.849   0.000   1.370  2
6   15.068  2.740   16.438  8.219   57.534 2
7   67.123  0.000   0.000   0.000   32.877 3
8   72.603  6.849   5.479   0.000   15.068 3
9   56.164  12.329  6.849   4.110   20.548 3
10  50.685  4.110   8.219   1.370   35.616 3
}\datatable

\begin{document}
\makeatletter
\begin{tikzpicture}
\begin{axis}[
    ylabel=label,
    xtick=data,
    xticklabels={S1,S2,S3,S4,S5,S6,S7,S8,S9,S10},
    enlarge y limits=false,
    enlarge x limits=0.1,
    ymin=0,ymax=100,
    ybar stacked,
    bar width=10pt,
    legend style={
      font=\footnotesize,
      cells={anchor=west},
      legend columns=5,
      at={(0.5,-0.2)},
      anchor=north,
      /tikz/every even column/.append style={column sep=0.2cm}
    },
    draw group line={[index]6}{1}{X Group}{-3.5ex}{7pt},
    draw group line={[index]6}{2}{Y Group}{-3.5ex}{7pt},
    draw group line={[index]6}{3}{Z Group}{-3.5ex}{7pt}
]

\addplot table[x index=0,y index=1] \datatable;
\addplot table[x index=0,y index=2] \datatable;
\addplot table[x index=0,y index=3] \datatable;
\addplot table[x index=0,y index=4] \datatable;
\addplot table[x index=0,y index=5] \datatable;
\legend{Far,Near,Here,There,NotThere}
\end{axis}
\end{tikzpicture}
\end{document}