Using \foreach to add rows containing references to pgfplots objects in a matrix of nodes

Question

This is a follow-up question to Using \pgfplotsinvokeforeach to create rows in a matrix of nodes. Consider the minimal working example below:

\documentclass{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.17, filter discard warning=false}
\usepackage{tikz}
\usetikzlibrary{matrix}
\begin{document}
\begin{tikzpicture}
\begin{axis}
  \addplot {2*x + 1};
  \label{myPlotA}
\addplot {3*x + 3};
  \label{myPlotB}
\addplot {x + 3};
  \label{myPlotC}
\matrix (myMatrix) [ matrix of nodes, at={(4,-5)} ] {
    \foreach\myletter in {A,B,C}{
      \ref{myPlot\myletter} \myletter
    } \
    %\ref{myPlotA} A \
    %\ref{myPlotB} B \
    %\ref{myPlotC} C \
  };
\end{axis}
\end{tikzpicture}
\end{document}

As written, the output looks like this but I am interested in producing the behavior given in the commented portion, i.e., output that looks like this in an automated way using \foreach or a similar command. I run into trouble because inserting the \\ needed to break between matrix rows causes a compilation error inside of the \foreach loop.

schtandard · Accepted Answer · 2022-08-19T08:46:20.280

You can build up a scratch variable first and then expand it to the whole matrix content at once. The scratch variable has to be global because \foreach uses a group for every iteration.

\documentclass{article}
\usepackage{etoolbox}
\usepackage{pgfplots}
\pgfplotsset{compat=1.17}
\usepackage{tikz}
\usetikzlibrary{matrix}
\begin{document}
\begin{tikzpicture}
\begin{axis}
  \addplot {2x + 1};
  \label{myPlotA}
  \addplot {3x + 3};
  \label{myPlotB}
  \addplot {x + 3};
  \label{myPlotC}
  \makeatletter
    \let@gtempa@empty
    \foreach\myletter in {A,B,C} {
      \xappto@gtempa{\noexpand\ref{myPlot\myletter} \myletter\noexpand\}
    }
    \matrix (myMatrix) [ matrix of nodes, at={(4,-5)} ] {
      @gtempa
    };
  \makeatother
\end{axis}
\end{tikzpicture}
\end{document}

However, I don't see why you wouldn't just use pgfplots' built-in legend capabilities.

\documentclass{article}
\usepackage{pgfplots}
\pgfplotsset{compat=1.17}
\begin{document}
\begin{tikzpicture}
\begin{axis} [
      legend style={
        draw=none,
        at={(axis cs:4,-5)},
        anchor=center,
      },
    ]
  \addplot {2x + 1};
  \addlegendentry{A}
  \addplot {3x + 3};
  \addlegendentry{B}
  \addplot {x + 3};
  \addlegendentry{C}
\end{axis}
\end{tikzpicture}
\end{document}

(If you only want to put the legend somewhere in that corner you can just use legend pos=south east.)

Using \foreach to add rows containing references to pgfplots objects in a matrix of nodes

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