How can a title be placed for a group of pgfplots?

Question

Using the groupplots library in pgfplots, how can a title be set for a group of plots? The following code (basically from the manual) is an example layout. I don't want each plot to have a title, but rather a title be placed centered on top of the group.

\documentclass[crop,tikz]{standalone}

\usepackage{pgfplots}
\usepgfplotslibrary{groupplots}

\begin{document}

\begin{tikzpicture}
  \begin{groupplot}[
    group style={
      {group size=2 by 2}},
    height=3cm,width=3cm,
    title={Title}]

    \nextgroupplot
    \addplot coordinates {(0,0) (1,1) (2,2)};
    \nextgroupplot
    \addplot coordinates {(0,2) (1,1) (2,0)};
    \nextgroupplot
    \addplot coordinates {(0,2) (1,1) (2,1)};
    \nextgroupplot
    \addplot coordinates {(0,2) (1,1) (1,0)};
  \end{groupplot}
\end{tikzpicture}

\end{document}

Answer 1

You can use a TikZ node as a title for a quick solution.

\documentclass{article}
\usepackage{pgfplots}
\usetikzlibrary{calc}
\usepgfplotslibrary{groupplots}
\begin{document}
\begin{tikzpicture}
  \begin{groupplot}[group style={group size=2 by 2},height=3cm,width=3cm]
    \nextgroupplot[title=One]
    \addplot coordinates {(0,0) (1,1) (2,2)};
    \nextgroupplot[title=Two]
    \addplot coordinates {(0,2) (1,1) (2,0)};
    \nextgroupplot[title=Three]
    \addplot coordinates {(0,2) (1,1) (2,1)};
    \nextgroupplot[title=Four]
    \addplot coordinates {(0,2) (1,1) (1,0)};
  \end{groupplot}
\node (title) at ($(group c1r1.center)!0.5!(group c2r1.center)+(0,2cm)$) {THE Title};
\end{tikzpicture}
\end{document}