PGFplots: Global setting of coordinates labels

Question

The following code results in an error when introducing the foreach part. So, I would like to know what is wrong in it.

\documentclass[border=1cm]{standalone}
\usepackage{pgfplots}

\pgfplotsset{compat=newest,
poles/.style= { only marks, mark=x, mark size = 1ex, thick},
zeros/.style= { only marks, mark=o, mark size = 1ex, thick }
}

\begin{document} 
\begin{tikzpicture}
    \begin{axis}[axis lines=middle,
    xmax = 1, xmin = -8,
    xtick={1,...,-9},
    ymax = 2, ymin = -2,
    ytick={-2,...,2},
    ]
        \addplot[poles] coordinates {(-2,2) (-2,-2) (-8,0)};
        \addplot[zeros] coordinates {(-4,0)};
        \foreach \x/\y in {-2/2 , -2/-2 , -8/0 , -4/0 }{
        \node[label={180:{(\x,\y)}},inner sep=2pt] at (axis cs:\x,\y) {};
     }
    \end{axis}
\end{tikzpicture}
\end{document}

---

Additionally, how to remove the foreach code and replace it with a global setting of coordinates labeling inside pgfplotsset with a local input option of the angular placement instead of the fixed angle 180 provided in my code?

Answer 1

Yes, this is one of the well-known expansion issue of pgfplots. Depending on how one views things, one may regard your question as a duplicate of the one that lead to this answer. If you feel it is a duplicate, I'll be happy to remove this answer. Otherwise just do

\documentclass[border=1cm]{standalone}
\usepackage{pgfplots}

\pgfplotsset{compat=newest,
poles/.style= { only marks, mark=x, mark size = 1ex, thick},
zeros/.style= { only marks, mark=o, mark size = 1ex, thick }
}

\begin{document} 
\begin{tikzpicture}
    \begin{axis}[axis lines=middle,clip=false,%<-added
    xmax = 1, xmin = -8,
    xtick={1,...,-9},
    ymax = 2, ymin = -2,
    ytick={-2,...,2},
    ]
        \addplot[poles] coordinates {(-2,2) (-2,-2) (-8,0)};
        \addplot[zeros] coordinates {(-4,0)};
        \foreach \x/\y/\z in {-2/2/-90 , -2/-2/90 , -8/0/90 , -4/0/90 }{
        \edef\temp{\noexpand\node[label={\z:{(\x,\y)}},inner sep=2pt] at (axis
        cs:\x,\y) {};}
        \temp
     }
    \end{axis}
\end{tikzpicture}
\end{document}

Why is that necessary? pgfplots runs a "survey phase" before it actually typesets things. And during this phase it can, in a way, forget things inside loops because there is another expansion magic taking place. For that reason, pgfplots comes with its own loop mechanism like \pgfplotsinvokeforeach and \pgfplotsforeachungrouped, see section 8.1 of the manual for more detail. My personal bottomline is to try out the above trick first, and if it works, then I am fine with it. ;-)

ADDENDUM: Here is an ugly version that allows you to add the anchors via point meta. (I have not much time now.) It works, but you can only have the anchors between 0 and 360, i.e. say -90 won't work. And there is an empty plot. The reason for this is that I have not the time now to find out how to do a better \pgfplotspointmetatransformed conversion.

\documentclass[border=1cm]{standalone}
\usepackage{pgfplots}

\pgfplotsset{compat=newest,
poles/.style= { scatter,
        scatter src=explicit, only marks, mark=x, mark size = 1ex, thick},
zeros/.style= { scatter,
        scatter src=explicit, only marks, mark=o, mark size = 1ex, thick }
}
\newcommand\myparse[1]{\pgfmathparse{#1}\typeout{\pgfmathresult}}
\begin{document} 
\begin{tikzpicture}
    \begin{axis}[axis lines=middle,clip=false,%<-added
    xmax = 1, xmin = -8,
    xtick={1,...,-9},
    ymax = 2, ymin = -2,
    ytick={-2,...,2},
    visualization depends on=x \as \rawx,
    visualization depends on=y \as \rawy,
    visualization depends on=\pgfplotspointmetatransformed \as \mymeta,
    nodes near coords style={
                anchor=(\pgfplotspointmetatransformed/1000)*360
            },
    nodes near coords={(\pgfmathprintnumber{\rawx},\pgfmathprintnumber{\rawy})},
    ]
        \addplot[poles,forget plot,opacity=0] coordinates {(0,0)[0] (0,0)[360]};
        \addplot[poles] coordinates {(-2,2)[90] (-2,-2)[270] (-8,0)[0]};
        \addplot[zeros] coordinates {(-4,0)[270]};
    \end{axis}
\end{tikzpicture}
\end{document}