用户土拨鼠发布了一个很好的方法，将图例项放在多图图表下方的一个框中，方法是将它们放在里面\node。
基于这个初始情况，我尝试将图例项分成三行，但结果却具有误导性。
最小工作示例（MWE）：
\documentclass[border=5mm]{standalone}
\usepackage{pgfplots}
\usetikzlibrary{positioning}
\pgfplotsset{compat=newest, scale only axis}
\begin{document}
\begin{tikzpicture}
\begin{scope}[local bounding box=plots]
\begin{axis}[
axis y line* = left,
legend to name = legA,
legend style = {draw=none,
legend columns = 3,
/tikz/every even column/.append style = {column sep=0.5cm},
/tikz/every odd column/.append style = {column sep=0.15cm},
},
]%
\addplot coordinates {(0,0) (1,1)};
\addplot coordinates {(1,0) (0,1)};
\addplot coordinates {(0,1) (1,1)};
\legend{Graph 1, Graph 2, Graph 3};
\end{axis}%
%
\begin{axis}[
axis x line = none,
axis y line* = right,
legend to name = legB,
legend style = {draw=none,
legend columns = 3,
/tikz/every even column/.append style = {column sep=0.5cm},
/tikz/every odd column/.append style = {column sep=0.15cm},
},
]%
\addplot coordinates {(0,1) (1,0)};
\addplot coordinates {(0,0.5) (1,0.5)};
\addplot coordinates {(0,0.15) (1,0.15)};
\legend{Graphinggraph 4, Graphinggraph 5, Graphinggraph 6};
\end{axis}%
%
\begin{axis}[
axis x line = none,
axis y line* = right,
legend to name = legC,
legend style = {draw=none,
legend columns = 3,
/tikz/every even column/.append style = {column sep=0.5cm},
/tikz/every odd column/.append style = {column sep=0.15cm},
},
]%
\pgfplotsset%
{%
every outer y axis line/.style = {xshift=2cm},
every tick/.style = {xshift=2cm},
every y tick label/.style = {xshift=2cm},
}%
\addplot coordinates {(0.5,0) (0.5,1)};
\addplot coordinates {(0.6,0) (0.6,1)};
\addplot coordinates {(0.7,0) (0.7,1)};
\legend{Graphgraph 7, Graphgraph 8, Graphgraph 9};
\end{axis}%
\end{scope}
\node[below=3.5em of plots.south] (legA) {\ref{legA}};
\node[below=-0.5em of legA] (legB) {\ref{legB}};
\node[below=-0.5em of legB] (legC) {\ref{legC}};
\draw (legA.north west) rectangle (legC.south east);
\end{tikzpicture}
\end{document}
结果截图：
问题说明：
如你看到的，
- 边框与图例项的外部尺寸不匹配，
- 图例项并未彼此精确对齐。
所以我的问题是：如何解决这个问题并让它看起来美观？
答案1
你可以text width给所有这些图例添加一个足够大的值。确实，这并不是完全自动的（Ti钾Z 确实理解类似的内容（width("blablabla")但在这里编写全自动解决方案可能有点小题大做），但对我来说似乎相当简单。
\documentclass[border=5mm]{standalone}
\usepackage{pgfplots}
\usetikzlibrary{positioning}
\pgfplotsset{compat=newest, scale only axis}
\begin{document}
\pgfplotsset{superb legend/.style={legend style = {draw=none,
legend columns = 3,
/tikz/every even column/.append style = {column sep=0.5cm,
text width=7em},
/tikz/every odd column/.append style = {column sep=0.15cm,
text width=7em},
}}}
\begin{tikzpicture}
\begin{scope}[local bounding box=plots]
\begin{axis}[
axis y line* = left,
legend to name = legA,
superb legend
]%
\addplot coordinates {(0,0) (1,1)};
\addplot coordinates {(1,0) (0,1)};
\addplot coordinates {(0,1) (1,1)};
\legend{Graph 1, Graph 2, Graph 3};
\end{axis}%
%
\begin{axis}[
axis x line = none,
axis y line* = right,
legend to name = legB,
superb legend
]%
\addplot coordinates {(0,1) (1,0)};
\addplot coordinates {(0,0.5) (1,0.5)};
\addplot coordinates {(0,0.15) (1,0.15)};
\legend{Graphinggraph~4, Graphinggraph~5, Graphinggraph~6};
\end{axis}%
%
\begin{axis}[
axis x line = none,
axis y line* = right,
legend to name = legC,
superb legend
]%
\pgfplotsset%
{%
every outer y axis line/.style = {xshift=2cm},
every tick/.style = {xshift=2cm},
every y tick label/.style = {xshift=2cm},
}%
\addplot coordinates {(0.5,0) (0.5,1)};
\addplot coordinates {(0.6,0) (0.6,1)};
\addplot coordinates {(0.7,0) (0.7,1)};
\legend{Graphgraph 7, Graphgraph 8, Graphgraph 9};
\end{axis}%
\end{scope}
\node[below=3.5em of plots.south] (legA) {\ref{legA}};
\node[below=-0.5em of legA] (legB) {\ref{legB}};
\node[below=-0.5em of legB] (legC) {\ref{legC}};
\draw (legA.north west) rectangle (legC.south east);
\end{tikzpicture}
\end{document}
（只是为了澄清：是的，我知道存在cycle list允许人们使所有情节风格不同的方法，但我不确定这里是否需要这样做。）
编辑：增加了text width，非常感谢 quark67！