pgfplots: Weird problem when using `\addlegendentry` with line break

Question

Bug/Issue Report: See here.

---

• I want to have an additional multi-line custom entry in the legend (\addlegendentry).
• Therefore, I follow Add custom entry into legend in pgfplot.
• I also want the legend to be left-aligned.
• Somehow, I need both (seemingly redundant) following lines to make it work:

---

legend style = {
    cells = {align = left}, % When commented out then error occurs
    },          
legend cell align = {left}, 

---

• If I remove cells = {align = left}, the I get an error.
• If I remove legend cell align = {left}, then the alignment is centered.
• Can you recreate the problem? Is this a bug that I should report?

---

\documentclass[12pt,letterpaper,landscape]{article}
\usepackage{mathtools}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}
\begin{document}
\begin{tikzpicture}
    \begin{axis}[
    legend style = {
        cells = {align = left}, % When commented out then error occurs
        },

    legend cell align = {left},

    ]
    % Plot A
    \addplot{x^2 - x + 4};
    \addlegendentry{$T_\text{R, I}$}
    % ---
    % https://tex.stackexchange.com/questions/204395/
    \addlegendimage{empty legend}
    \addlegendentry{
        Line 1\
        Line 2
        }

    \end{axis}
\end{tikzpicture}
\end{document}

Error Message

Missing } inserted.
<inserted text> 
                }
l.27  \end{axis}

Answer 1

I can reproduce the error, might be worth a bug report.

---

The manual says in the description of legend cell align that

the choice left is equivalent to legend style={cells={anchor=west}}

And if you do

legend style = {cells = {align = left, anchor=west}},

while removing legend cell align=left, it seems to work as desired.

\documentclass[12pt,letterpaper,landscape]{article}
\usepackage{mathtools}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}
\begin{document}
\begin{tikzpicture}
    \begin{axis}[
    legend style = {cells = {align = left, anchor=west}},
    ]
    % Plot A
    \addplot{x^2 - x + 4};
    \addlegendentry{$T_\text{R, I}$}
    % ---
    % https://tex.stackexchange.com/questions/204395/
    \addlegendimage{empty legend}
    \addlegendentry{
        Line 1\
        Line 245
        }

%    \addlegendimage{empty legend}
%    \addlegendentry{a}
\end{axis}

\end{tikzpicture}
\end{document}

Answer 2

This is not a bug, but intended behavior. See the comments in the following code for some explanations.

\documentclass[border=5pt]{standalone}
\usepackage{mathtools}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}
\begin{document}
\begin{tikzpicture}
    \begin{axis}[
        % this key manages how *different* legend entries are aligned to each other
        legend cell align=right,
        % this key manages how *multi-line* text in a *single* legend entry is aligned.
        % You receive an error because it is *required* to set the `align` option
        % when you have multi-line text. ...
        legend style={cells={align=left}}, % When commented out then error occurs
    ]
        \addplot{x^2 - x + 4};
        \addlegendentry{$T_\text{R, I}$}
        \addlegendimage{empty legend}
        \addlegendentry{
            Line 1 \\
            Line number two
        }
        \addlegendimage{empty legend}
        \addlegendentry{just some long text}
    \end{axis}
% ... You'll get the same error if you comment the `align` option here,
% i.e. this is standard (intended) TikZ behavior.
\node at (1,1) [
    align=center,
] {some \\ text};


\end{tikzpicture}
\end{document}