Alignment / scaling of pgfplot legends

Question

I would like to align two plots containing legends on their outside:

\documentclass[a4paper]{article}

\usepackage{tikz}
\usepackage{tikzscale}
\usepackage{subcaption}
\usepackage{pgfplots}
\pgfplotsset{compat=1.13}

\begin{document}
\begin{figure}
  \begin{centering}
    \begin{subfigure}[t]{0.4\textwidth}
      \vskip 0pt
      \includegraphics[width=\textwidth]{plot1.tikz}
    \end{subfigure}
    ~
    \begin{subfigure}[t]{0.4\textwidth}
      \vskip 0pt
      \includegraphics[width=\textwidth]{plot2.tikz}
    \end{subfigure}
  \end{centering}
\end{figure}
\end{document}

Specifically, the two plot files contain the following:

% plot1.tikz
  \begin{tikzpicture}
    \begin{axis}[
        legend entries={Short caption},
        legend style = {at = {(0.5, -0.3)}, anchor = north},
        width=\textwidth,
        height=\textwidth]
      \addplot[color=red,mark=x] coordinates {
        (2,-2.8559703)
        (8,-7.9377747) };
    \end{axis}
  \end{tikzpicture}

% plot2.tikz
\begin{tikzpicture}
  \begin{axis}[
      legend entries={Long long long long long long caption},
      legend style = {at = {(0.5, -0.3)}, anchor = north},
      width=\textwidth,
      height=\textwidth]
    \addplot[color=red,mark=x] coordinates {
      (2,-2.8559703)
      (8,-7.9377747) };
  \end{axis}
\end{tikzpicture}

Unfortunately, the width of the caption of the second plot dominates the width of the plot itself. Therefore, the plot is shrunk and the sizes don't match up anymore:

I would like the layout of the legend to break this long line and to maintain a fixed width. How can I achieve that effect?

Is there a way to do so?

Answer 1

You can specify line breaks using \\, but in addition you must modify the legend style and specify a location to align, for example cells={align=left}.

As an aside, note that \centering is a switch command and isn't written as \begin{centering}...\end{centering}. I fixed that for you.

\documentclass[a4paper]{article}
\usepackage{tikz}
\usepackage{tikzscale}
\usepackage{subcaption}
\usepackage{pgfplots}
\pgfplotsset{compat=1.13}

\usepackage{filecontents} % <--- To make a self contained MWE

\begin{filecontents*}{plot1.tikz}
  \begin{tikzpicture}
    \begin{axis}[
        legend entries={Short caption},
        legend style = {at = {(0.5, -0.3)}, anchor = north},
        width=\textwidth,
        height=\textwidth]
      \addplot[color=red,mark=x] coordinates {
        (2,-2.8559703)
        (8,-7.9377747) };
    \end{axis}
  \end{tikzpicture}
\end{filecontents*}

\begin{filecontents*}{plot2.tikz}
\begin{tikzpicture}
  \begin{axis}[
      legend entries={Long long long \\ long long long \\ caption},
      legend style = {at = {(0.5, -0.3)}, anchor = north,cells={align=left}},
      width=\textwidth,
      height=\textwidth]
    \addplot[color=red,mark=x] coordinates {
      (2,-2.8559703)
      (8,-7.9377747) };
  \end{axis}
\end{tikzpicture}
\end{filecontents*}

\begin{document}
\begin{figure}
  \centering  % <--- Fixed
    \begin{subfigure}[t]{0.4\textwidth}
      \vskip 0pt
      \includegraphics[width=\textwidth]{plot1.tikz}
    \end{subfigure}
    ~
    \begin{subfigure}[t]{0.4\textwidth}
      \vskip 0pt
      \includegraphics[width=\textwidth]{plot2.tikz}
    \end{subfigure}
\end{figure}
\end{document}