pgfplots ：图例中的行标题

Question

我想在 pgfplots 中为图例的每一行添加一个标题。我通过以下示例实现了此目的：

\documentclass{article}
\usepackage[pdftex, active, floats, tightpage]{preview}
\usepackage[utf8x]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}

\begin{document}

\begin{figure}
\begin{tikzpicture}

    \begin{axis}[
     axis on top,
     axis equal image,
     xmin=-1, xmax=1.05, ymin=-0.05, ymax=1.5
    ,axis x line=bottom, axis y line=left
    , legend columns=3, legend style={cells={anchor=west}, at={(0.5,1)}, anchor=north, draw=none, name=legend_name
    ,width=\textwidth
    ]

    \addplot [color=blue, thick, mark=none, domain=-1:1, samples=100]  {x^2};
    \addlegendentry{$x^2$}
    \addplot [color=blue, thick, loosely dashed, mark=none, domain=-1:1, samples=100] {x^3};
    \addlegendentry{$x^3$}
    \addplot [color=blue, thick, dotted, mark=none, domain=-1:1, samples=100] {x^4};
    \addlegendentry{$x^4$}

    \addplot [color=red, thick, mark=none, domain=-1:1, samples=100] {2*x^2};
    \addlegendentry{$2 x^2$}
    \addplot [color=red, thick, loosely dashed, mark=none, domain=-1:1, samples=100] {2*x^3};
    \addlegendentry{$2 x^3$}
    \addplot [color=red, thick, dotted, mark=none, domain=-1:1, samples=100] {2*x^4};
    \addlegendentry{$2 x^4$}

    \end{axis}

    \node (legend_row_title) at (legend_name.west) [anchor=east, align=center, xshift=2pt] {1\textsuperscript{st} row :\\2\textsuperscript{nd} row :};
    \draw (legend_row_title.north west) rectangle (legend_name.south east);

\end{tikzpicture}
\end{figure}

\end{document}

但问题是，用于对齐图例的锚点是图例本身的中心，因此矩形和行标题对齐不正确。有人知道如何正确对齐吗？或者至少修复对齐问题？谢谢！

Answer 1

您可以使用

 \addlegendimage{empty legend}
 \addlegendentry{1\textsuperscript{st} row :}

在适当的位置添加虚拟图例：

\documentclass{article}
\usepackage[pdftex, active, floats, tightpage]{preview}
\usepackage[utf8x]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}

\begin{document}

\begin{figure}
\begin{tikzpicture}

    \begin{axis}[
     axis on top,
     axis equal image,
     xmin=-1, xmax=1.05, ymin=-0.05, ymax=1.5
    ,axis x line=bottom, axis y line=left
    , legend columns=4, legend style={cells={anchor=west}, at={(0.5,1)}, anchor=north, draw=none, name=legend_name,draw}
    ,width=\textwidth
    ]
    \addlegendimage{empty legend}
     \addlegendentry{1\textsuperscript{st} row :}
    \addplot [color=blue, thick, mark=none, domain=-1:1, samples=100]  {x²};
    \addlegendentry{$x²$}
    \addplot [color=blue, thick, loosely dashed, mark=none, domain=-1:1, samples=100] {x³};
    \addlegendentry{$x³$}
    \addplot [color=blue, thick, dotted, mark=none, domain=-1:1, samples=100] {x⁴};
    \addlegendentry{$x⁴$}
    \addlegendimage{empty legend}
    \addlegendentry{2\textsuperscript{nd} row :}
    \addplot [color=red, thick, mark=none, domain=-1:1, samples=100] {2*x²};
    \addlegendentry{$2 x²$}
    \addplot [color=red, thick, loosely dashed, mark=none, domain=-1:1, samples=100] {2*x³};
    \addlegendentry{$2 x³$}
    \addplot [color=red, thick, dotted, mark=none, domain=-1:1, samples=100] {2*x⁴};
    \addlegendentry{$2 x⁴$}

    \end{axis}

    %\node (legend_row_title) at (legend_name.west) [text width=4em,anchor=east, align=right, xshift=2pt] {\strut 1\textsuperscript{st} row :\\2\textsuperscript{nd} row :};
%    \draw (legend_row_title.north west) rectangle (legend_name.south east);

\end{tikzpicture}
\end{figure}

\end{document}