pgfplots 中一组水平对齐的图形下方的共享图例

Question

我应该如何运行以下示例代码来/pgfplots/legend to name={<name>}将共享图例放在一组水平对齐的图形下方？

/pgfplots/legend to name={<name>}手册中“轴外的图例”下使用的示例代码pgfplots如下（前两行是我自己添加的）。

\documentclass{standalone}
\usepackage{pgfplots}
% Preamble: \pgfplotsset{width=7cm,compat=1.3}
\begin{document}
\pgfplotsset{footnotesize,samples=10}
\begin{center}% note that \centering uses less vspace...
\begin{tikzpicture}
\begin{axis}[
legend columns=-1,
legend entries={$(x+0)^k$;,$(x+1)^k$;,$(x+2)^k$;,$(x+3)^k$},
legend to name=named,
title={$k=1$}]
\addplot {x};
\addplot {x+1};
\addplot {x+2};
\addplot {x+3};
\end{axis}
\end{tikzpicture}
%
\begin{tikzpicture}
\begin{axis}[title={$k=2$}]
\addplot {x^2};
\addplot {(x+1)^2};
\addplot {(x+2)^2};
\addplot {(x+3)^2};
\end{axis}
\end{tikzpicture}
%
\begin{tikzpicture}
\begin{axis}[title={$k=3$}]
\addplot {x^3};
\addplot {(x+1)^3};
\addplot {(x+2)^3};
\addplot {(x+3)^3};
\end{axis}
\end{tikzpicture}
\\

\ref{named}
\end{center}
\end{document}

它应该生成共享图例以下一组三个水平对齐的图形：

然而，它会抛出错误 Something is wrong -- perhaps a missing \item，并生成一个共享的图例向右。

编译器将此归咎于begin{center} end{center}环境。但是，注释掉它们却没有帮助。

Answer 1

下列的答案由@LaRiFaRi 提供，groupplots对我有用（ShareLatex 上的代码)：

\documentclass{standalone}
\usepackage{pgfplots}
\usetikzlibrary{pgfplots.groupplots}
\pgfplotsset{compat=1.3}
\begin{document}
  \begin{tikzpicture}
    \pgfplotsset{footnotesize,samples=10}
    \begin{groupplot}[group style = {group size = 3 by 1, horizontal sep = 50pt}, width = 6.0cm, height = 5.0cm]
        \nextgroupplot[ title = {$k=1$},
            legend style = { column sep = 10pt, legend columns = -1, legend to name = grouplegend,}]
            \addplot {x};   \addlegendentry{$(x+0)^k$}%
            \addplot {x+1}; \addlegendentry{$(x+1)^k$}
            \addplot {x+2}; \addlegendentry{$(x+2)^k$}
            \addplot {x+3}; \addlegendentry{$(x+3)^k$}
        \nextgroupplot[title = {$k=2$},]
            \addplot {x^2};
            \addplot {(x+1)^2};
            \addplot {(x+2)^2};
            \addplot {(x+3)^2};
        \nextgroupplot[title = {$k=3$},] 
            \addplot {x^3};
            \addplot {(x+1)^3};
            \addplot {(x+2)^3};
            \addplot {(x+3)^3};                    
    \end{groupplot}
    \node at ($(group c2r1) + (0,-4.0cm)$) {\ref{grouplegend}}; 
\end{tikzpicture}
\end{document}