What is the difference between storing table data by filecontents and pgfplotstableread?

Question

In this MWE, the table data loaded from poles.dat is successfully parsed as shown in the left plot.

However, storing the same data inside a macro using \pgfplotstableread doesn't result in the same expected output.

Why is there a difference between both ways? and how to make the second approach of pgfplotstableread work like the first one of *.dat file?

\documentclass[border=1cm]{standalone}

\usepackage{pgfplots,pgfplotstable}

\pgfplotsset{compat=newest,
    poles/.style= { only marks, mark=x, mark size = 1ex, thick},
    point meta = explicit symbolic,
    visualization depends on={\thisrow{angle} \as \myangle},
    visualization depends on={value \thisrow{label} \as \mylabel},
    Label Style/.style args = {#1}{
        nodes near coords,
        every node near coord/.style = %
        {
            anchor=south, label={[#1]\myangle:{\mylabel}}
        },
    }
}

\usepackage{filecontents}
\begin{filecontents*}{poles.dat}
    Re      Im      label       angle
    -2      2       (-2,2)      270
    -2      -2      (-2,-2)     90
    -4      0       (-4,0)      60
\end{filecontents*}

\pgfplotstableread{
    Re      Im      label       angle
    -2      2       (-2,2)      270
    -2      -2      (-2,-2)     90
    -4      0       (-4,0)      60
}\mytable

\begin{document} 
    \begin{tikzpicture}
    \begin{axis}
        \addplot[Label Style={blue,fill = gray!20},poles]   table {poles.dat};
    \end{axis}
    \end{tikzpicture}
    \begin{tikzpicture}
    \begin{axis}
        \addplot[Label Style={blue,fill = gray!20},poles]   table {\mytable};
    \end{axis}
    \end{tikzpicture}
\end{document}

Answer 1

I found the following in the pgfplots source code.

% The normal implementation of \thisrow is not accessable here. And the
% worst is: error messages are impossible either because they are
% not executed... we resort to the associated math functions:
\def\thisrow##1{thisrow("##1")}% let us hope that math parsing is active!

Note that this solution always uses \mytable. The key was the use of \coordindex.

\documentclass[border=1cm]{standalone}

\usepackage{pgfplotstable,pgfplots}

\pgfplotsset{compat=newest,
    poles/.style= { only marks, mark=x, mark size = 1ex, thick},
    point meta = explicit symbolic,
    visualization depends on={\thisrow{angle} \as \myangle},
    %visualization depends on={value \thisrow{label} \as \mylabel},
    Label Style/.style args = {#1}{
        nodes near coords,
        every node near coord/.style = %
        {
            anchor=south, label={[#1]\myangle:{\pgfplotstablegetelem{\coordindex}{label}\of{\mytable}\pgfplotsretval}}
        },
    }
}
\newcommand{\myotherlabel}{\pgfmathparse{\thisrow{label}}\pgfmathresult}

\usepackage{filecontents}
\begin{filecontents*}{poles.dat}
    Re      Im      label       angle
    -2      2       (-2,2)      270
    -2      -2      (-2,-2)     90
    -4      0       (-4,0)      60
\end{filecontents*}

\pgfplotstableread[columns/label/.style={string}]{
    Re      Im      label       angle
    -2      2       (-2,2)      270
    -2      -2      (-2,-2)     90
    -4      0       (-4,0)      60
}\mytable

\begin{document} 
    \begin{tikzpicture}
    \begin{axis}
        \addplot[Label Style={blue,fill = gray!20},poles]   table {poles.dat};
    \end{axis}
    \end{tikzpicture}
    \begin{tikzpicture}
    \begin{axis}
        \addplot[Label Style={blue,fill = gray!20},poles]   table {\mytable};
    \end{axis}
    \end{tikzpicture}
\end{document}

Answer 2

Here is a crazy workaround: write the table to a file and read it again. (Of course, in the present example this does not make sense.) However, there are situations in which it can make sense, e.g. when you create the table through macros or when you really want to skip some rows, like here.

\documentclass[border=1cm]{standalone}

\usepackage{pgfplots,pgfplotstable}

\pgfplotsset{compat=newest,
    poles/.style= { only marks, mark=x, mark size = 1ex, thick},
    point meta = explicit symbolic,
    visualization depends on={\thisrow{angle} \as \myangle},
    visualization depends on={value \thisrow{label} \as \mylabel},
    Label Style/.style args = {#1}{
        nodes near coords,
        every node near coord/.style = %
        {
            anchor=south, label={[#1]\myangle:{\mylabel}}
        },
    }
}

% from https://tex.stackexchange.com/a/445369/121799
\newcommand*{\ReadOutElement}[4]{%
    \pgfplotstablegetelem{#2}{#3}\of{#1}%
    \let#4\pgfplotsretval
}
% based on https://tex.stackexchange.com/a/307032/121799
% and https://tex.stackexchange.com/a/451326/121799
\newcommand{\GetRow}[2]{
\pgfplotstablegetcolsof{\mytable}
\pgfmathtruncatemacro{\colnumber}{\pgfplotsretval-1}
\foreach \XX in {0,...,\colnumber}
{%
\ReadOutElement{\mytable}{#1}{[index]\XX}{\tmp}%
\ifnum\XX=0%
\xdef#2{{\tmp}}%
\else%
\xdef#2{#2,{\tmp}}%
\fi%
}
}

\usepackage{filecontents}
\begin{filecontents*}{poles.dat}
    Re      Im      label       angle
    -2      2       (-2,2)      270
    -2      -2      (-2,-2)     90
    -4      0       (-4,0)      60
\end{filecontents*}

\pgfplotstableread{
    Re      Im      label       angle
    -2      2       (-2,2)      270
    -2      -2      (-2,-2)     90
    -4      0       (-4,0)      60
}\mytable

\pgfplotstablegetrowsof{\mytable}%
\pgfmathtruncatemacro{\rownum}{\pgfplotsretval-1}%
\pgfplotstablegetcolsof{\mytable}%
\pgfmathtruncatemacro{\colnum}{\pgfplotsretval-1}%
\foreach \X in {0,...,\colnum}%
{\pgfplotstablegetcolumnnamebyindex{\X}\of{\mytable}\to\pgfplotsretval%
\ifnum\X=0%
\xdef\tmp{\pgfplotsretval}%
\else%
\xdef\tmp{\tmp,\pgfplotsretval}%
\fi}
\newwrite\myoutput% from https://tex.stackexchange.com/a/290058/121799
\immediate\openout\myoutput=\jobname-tmp.dat%
\immediate\write\myoutput{\tmp}% create table header
\foreach \X in {0,...,\rownum}% rows
{\GetRow{\X}{\myrow}%
\immediate\write\myoutput{\myrow}}%
\immediate\closeout\myoutput%

\begin{document} 
    \begin{tikzpicture}
    \begin{axis}
        \addplot[Label Style={blue,fill = gray!20},poles]   table {poles.dat};
    \end{axis}
    \end{tikzpicture}
    \begin{tikzpicture}
    \begin{axis}
        \addplot[Label Style={blue,fill = gray!20},poles]   
        table[col sep=comma] {\jobname-tmp.dat};
    \end{axis}
    \end{tikzpicture}
\end{document}

Needless to say that one could clean up the macros. I hesitate to do that before I know all possible applications. One is to skip rows, which is important for regression plots, and another one is like here in which one uses pgfplots to do more complicated annotations. I'm wondering if there are further obvious use cases.