Stack PGFPlots Vertically

stack_pgfplot.tex

\documentclass{article}

\usepackage{pgfplots}
\usetikzlibrary{pgfplots.groupplots}

\begin{document}
\begin{tikzpicture}
\begin{groupplot}[
    group style={
        group name=my plots,
        group size=1 by 3,
        xlabels at=edge bottom,
        xticklabels at=edge bottom,
        vertical sep=0pt
    },
    ybar,
    footnotesize,
    width=8cm,
    height=3cm,
    xlabel=Site,
    xmin=-0.5, xmax=10.5,
    ymin=0, ymax=29,
    xtick={0,1,...,12},
    xticklabels={A,B,C,D,E,F,G,H,I,J,K},
    tickpos=left,
    ytick align=outside,
    xtick align=outside
]
\nextgroupplot[ymax=11]
\addplot [fill=blue] coordinates{(0,0) (1,9.94) (2,0.72)(3,1.17)(4,1.05)(5,0.67)(6,0.24)(7,1.75)(8,0)(9,0)(10,1.21)(11,0)};
\nextgroupplot
\addplot [fill=red] coordinates{(0,0) (1,14.04) (2,0.72)(3,0)(4,22.81)(5,3.34)(6,2.05)(7,11.23)(8,2.59)(9,0)(10,0.61)(11,0)};
\nextgroupplot[ymax=5]
\addplot [fill=red] coordinates{(0,0) (1,0) (2,0)(3,0)(4,0.53)(5,0.33)(6,0.73)(7,0)(8,0)(9,0)(10,3.9)(11,0.17)};
\end{groupplot}
\end{tikzpicture}

\end{document}