Align legend text along a certain character within tikzpicture environment

Question

Suppose we have the following:

\documentclass{standalone}
\usepackage{tikz}
\usepackage{pgfplots}
\usepackage{pgfplotstable}
\pgfplotsset{compat=1.16}
\usepgfplotslibrary{external}
\tikzexternalize
\begin{document}
\begin{tikzpicture}
\begin{axis}[%legend cell align={&+},?
             legend entries={a+b, aaaa+bbb},]
    \addplot {x};
    \addplot {x^2};
\end{axis}
\end{tikzpicture}
\end{document}

Is it possible to align text in a legend with respect to a certain character? e.g. like we might do in an align environment

\begin{align}
    a &+ b\\
    aaaa &+ bbb
\end{align}

I know we can do legend cell align=left etc. but this will only work for the above case when the legend entries themselves meet a certain criteria.

As an aside, where do I find a comprehensive documentation page for pgfplots? I have been referring to this although it is not all inclusive.

Welcome to TeX.SX! To answer the second part of the question, you can find the documentation of pgfplots on CTAN, on this page. — Vincent, Jan 23 '20 at 03:24
You do realize that all externalize does is run TikZ using standalone in a separate process and use \includegraphics for the image file created. — John Kormylo, Jan 23 '20 at 15:53
Right... In the end I just need something like this and your solution works with externalize anyway, despite your chip. — algae, Jan 24 '20 at 01:19

score 1 · Answer 1 · answered Jan 23 '20 at 04:21

Here is one way to achieve this alignment.

\documentclass{standalone}
\usepackage{eqparbox}
\usepackage{pgfplots}
\usepackage{pgfplotstable}
\pgfplotsset{compat=1.16}
% \usepgfplotslibrary{external}
% \tikzexternalize
\begin{document}
\begin{tikzpicture}
\begin{axis}[%legend cell align={&+},?
             legend entries={\eqmakebox[r1][r]{$b$}\eqmakebox[l1][l]{${}+b$},
              \eqmakebox[r1][r]{$aaaa$}\eqmakebox[l1][l]{${}+bbb$}}]
    \addplot {x};
    \addplot {x^2};
\end{axis}
\end{tikzpicture}
\end{document}

One may simplify the usage. Whether there is a stable way to marry this to externalize I do not know.

Thanks, I'll accept soon if there are no suggestions on what to do with externalize. I'm very new to pgf but it seems all but the most trivial cases require the externalize option to have anything compile due to size limitations. i.e. imported data needs to be further discretized into smaller sets which is difficult for curves with fine structure. — algae, Jan 23 '20 at 04:29

score 1 · Accepted Answer · answered Jan 23 '20 at 04:34

Here is another way to do it. See also \mathrlap and \mathllap from mathtools.

\documentclass{standalone}
\usepackage{tikz}
\usepackage{pgfplots}
\usepackage{pgfplotstable}
\pgfplotsset{compat=1.16}
\usepgfplotslibrary{external}
%\tikzexternalize% sorry, not on my system
\begin{document}
\begin{tikzpicture}
\begin{axis}[%legend cell align={&+},?
             legend entries={\phantom{aaaa}\llap{a}+\rlap{b}\phantom{bbb}, aaaa+bbb},]
    \addplot {x};
    \addplot {x^2};
\end{axis}
\end{tikzpicture}
\end{document}

Align legend text along a certain character within tikzpicture environment

2 Answers2