multiple y axes in groupplot

Question

I want to create a barplot with three groups. Compared are two criteria: water consumption and CO2 emission. To make it more comprehensive here the plot I already generated

I approached this problem by using the example from Stacked barplots and text labels in x-coordinate. You can see that the waterconsumption has very small values in comparisson to the ones of the co2-emission. So the Question is:

How can I create a second y-axis for the values of the water consumption?

MWE:

\documentclass{article}
\usepackage{pgfplots,pgfplotstable}
\usetikzlibrary{pgfplots.groupplots}
\pgfplotsset{compat=1.4}

\pgfplotstableread{
Criterion   Wasserverbrauch   {CO$_2$-Emission}
Komp        8           2349
Komp+PV 8           452
Sorp        14          1006
}\Rapperswil

\pgfplotstableread{
Criterion   Wasserverbrauch   {CO$_2$-Emission}
Komp        36          10220
Komp+PV 36          5891
Sorp        85          3160
}\Palermo

\pgfplotstableread{
Criterion   Wasserverbrauch   {CO$_2$-Emission}
Komp        97          25657
Komp+PV 97          18306
Sorp        184         7461
}\Jeddah

\begin{document}

\begin{tikzpicture}
  \begin{groupplot}
    [group style= {
    columns=3,xlabels at=edge bottom,
    group name=plots,   y descriptions at=edge left,
    },
    ybar,
    ymin=0,
    ymax=27e3,
    enlarge x limits={abs=.5},
    width=0.45\textwidth,
    height=0.6\textwidth,
    scaled y ticks=base 10:-3,
    xticklabels from table={\Rapperswil}{Criterion},
    x tick label style={rotate=90,anchor=east},
    xtick=data,legend columns=2]

    \nextgroupplot[xlabel=Rapperswil,legend to name=grouplegend,ylabel={[kg CO$_2$ per year]}]
    \pgfplotstableforeachcolumn\Rapperswil\as\col{%
        \ifnum\pgfplotstablecol=0
        \else
        \edef\tmp{%
            \noexpand\addplot table [x expr=\noexpand\coordindex,y=\col] {\noexpand\Rapperswil};
            \noexpand\addlegendentry {\col}%
        }%
        \tmp
        \fi
    }

    \nextgroupplot[xlabel=Palermo]
    \pgfplotstableforeachcolumn\Palermo\as\col{%
        \ifnum\pgfplotstablecol=0 
        \else
        \edef\tmp{%
            \noexpand\addplot table [x expr=\noexpand\coordindex,y=\col] {\noexpand\Palermo};
        }%
        \tmp
        \fi
    }

    \nextgroupplot[xlabel=Jeddah]
    \pgfplotstableforeachcolumn\Jeddah\as\col{%
        \ifnum\pgfplotstablecol=0 
        \else
        \edef\tmp{%
            \noexpand\addplot table [x expr=\noexpand\coordindex,y=\col] {\noexpand\Jeddah};
        }%
        \tmp
        \fi
    }
  \end{groupplot}

  \node at (plots c2r1.north) [anchor=south, yshift=.6cm] {\ref{grouplegend}};
\end{tikzpicture}

\end{document}

Answer 1

The approach for combining different y-axes when using groupplots is very similar to that when using just one plot: You draw the second set of plots on top of the first one, hiding the axis lines and axis labels, and moving the y(tick)labels to the other side:

\documentclass{article}
\usepackage{pgfplots,pgfplotstable}
\usetikzlibrary{pgfplots.groupplots}
\pgfplotsset{compat=1.4}

\pgfplotstableread{
Criterion   Wasserverbrauch   {CO$_2$-Emission}
Komp        8           2349
Komp+PV 8           452
Sorp        14          1006
}\Rapperswil

\pgfplotstableread{
Criterion   Wasserverbrauch   {CO$_2$-Emission}
Komp        36          10220
Komp+PV 36          5891
Sorp        85          3160
}\Palermo

\pgfplotstableread{
Criterion   Wasserverbrauch   {CO$_2$-Emission}
Komp        97          25657
Komp+PV 97          18306
Sorp        184         7461
}\Jeddah

\begin{document}

\begin{tikzpicture}
\pgfplotsset{
    water/.style={
        fill=cyan,
        draw=cyan!50!black
    },
    co2/.style={
        fill=orange,
        draw=orange!50!black
    }
}

  \begin{groupplot}[
    group style={
        columns=3,
        group name=plots,
        x descriptions at=edge bottom,
        y descriptions at=edge left,
    },
    ylabel={[ML]}, ylabel style=cyan!50!black,
    yticklabel style=cyan!50!black,
    ybar, /pgf/bar shift=-6pt, % We have to manually move the bars
    ymin=0, ymax=270,
    enlarge x limits={abs=.5},
    width=0.4\textwidth,
    height=0.6\textwidth,
    xticklabels from table={\Rapperswil}{Criterion},
    x tick label style={rotate=90,anchor=east},
    xtick=data, xtick pos=left,
    legend columns=2]

    \nextgroupplot[xlabel=Rapperswil,legend to name=grouplegend]
    \addplot [water] table [x expr=\coordindex,y index=1] {\Rapperswil};
        \addlegendentry {Wasserverbrauch}%
    \addlegendimage{co2,ybar legend}
        \addlegendentry{CO$_2$-Emission}

    \nextgroupplot[xlabel=Palermo]
    \addplot [water] table [x expr=\coordindex,y index=1] {\Palermo}; 

    \nextgroupplot[xlabel=Jeddah]
     \addplot [water] table [x expr=\coordindex,y index=1] {\Jeddah};
  \end{groupplot}

  \begin{groupplot}
    [group style= {
        columns=3,
        group name=plots,
        y descriptions at=edge right,
    },
    ybar, /pgf/bar shift=6pt,
    ymin=0, ymax=2.7e4,
    enlarge x limits={abs=.5},
    width=0.4\textwidth,
    height=0.6\textwidth,
    xtick=\empty, axis line style=transparent,
    ylabel={[kg CO$_2$ per year]},yticklabel style=orange!75!black,
    ylabel style=orange!75!black]

    \nextgroupplot[scaled y ticks=false]
    \addplot [co2] table [x expr=\coordindex,y index=2] {\Rapperswil};

    \nextgroupplot[scaled y ticks=false]
     \addplot [co2] table [x expr=\coordindex,y index=2,
        ] {\Palermo}; 

    \nextgroupplot
     \addplot [co2] table [x expr=\coordindex,y index=2] {\Jeddah};
  \end{groupplot}

  \node at (plots c2r1.north) [anchor=south, yshift=.6cm] {\ref{grouplegend}};
\end{tikzpicture}

\end{document}