Problem: tikz, pgfplots and external coordinates using overlay

Question

I need to draw arrows from text to a pgfplots graph using TikZ. For some reason, the arrow coordinates are not where I want them to be in the pgfplots graph. Here is an example:

\documentclass[12pt]{article}
\usepackage{tikz}
\usepackage{pgfplots}

\pgfplotsset{width=7cm}

\begin{document}

Text \tikz[remember picture] \node[circle,fill=red!50] (n1) {}; text.
\vspace{1in}

\pgfplotsset{domain=-1:1}
\begin{tikzpicture}
  \begin{axis}[remember picture,xlabel=A normal sized $x$ label]
    \addplot[smooth,blue,mark=*] {x^2};
    \node (c) at (axis cs:0,0.5) {$a$};
  \end{axis}
\end{tikzpicture}
\vspace{1in}

Text \tikz[remember picture] \node[fill=blue!50] (n2) {}; text.

\begin{tikzpicture}[remember picture,overlay]
  \draw [->,red,very thick] (n1) -- (n2);
  \draw [overlay,->,very thick] (c) -- (n2);
\end{tikzpicture}

\end{document}

Letter 'a' seems to be in the correct place at (0,0.5), but the starting point of the black arrow is not. It seems to be too much to the right.

The red arrow, which is not defined using the pgfplots graph, seems to be correct.

Is there some option, which I have missed?

Thank you for any help!

Answer 1

This seems to be a bug in pgfplots, I will take care of it.

Although I do not see the mistake, I can say how to AVOID the problem until the bugfix is ready - provided you are running with pgfplots 1.6.1 (the most recent stable):

adding

\pgfplotsset{cell picture=if necessary}

to your preamble (or \begin{axis}[..., cell picture=if necessary] to selected axes) will fix it.

ATTENTION: in this case, you need to move remember picture from \begin{axis} to \begin{tikzpicture}!

You can also say cell picture=false. In this case you will receive error messages if pgfplots really needs the initial setting (which is cell picture=true as you might have guessed).

---

Technical explanation:

I see that it is somehow related to the fact that pgfplots creates a so-called "cell-picture" and moves it to implement its positioning choices (the anchors). A cell-picture means that it creates an additional \tikzpicture...\endtikzpicture internally just like a pgf matrix.

The initial value cell picture=true is mainly for backwards compatibility. In general, it has advantages to avoid cell pictures. They are necessary if you choose an anchor which is NOT in the list

north, north west, west, south west, south, south east, east, north east, north, center, origin, above origin, left of origin, right of origin, below origin.

For example, the choice anchor=above north requires a cell picture.

---

EDIT at July 2016

@ Elmar pointed out that the issue is not resolved completely and he provided the minimal example

%compile with: pdflatex -shell-escape main && make -f main.makefile && pdflatex main
\documentclass{article}
\usepackage{pgfplots}
\usetikzlibrary{external}
\tikzset{external/export=false, external/mode=list and make}
\tikzexternalize
\pgfplotsset{compat=1.13}
\begin{document}
    \tikz[remember picture] \node {X};
    \tikzset{external/export=true}
    \begin{tikzpicture}
        \begin{axis}[clip=false]
            \addplot {x};
            \node (A) at (yticklabel cs:0.5) {A};
        \end{axis};
        \node at (A) {B};
    \end{tikzpicture}
\end{document}

and he has a point - there are still issues. In his example, the problem is caused by an entry in the main.aux file which refers to the "remembered position" -- it is accidentally associated with the second image.

I will look into it. For now, a workaround appears to be to not optimize the first picture away as this will result in correct internal picture identifiers when externalizing the graphics:

%compile with: pdflatex -shell-escape main && make -f main.makefile && pdflatex main
\documentclass{article}
\usepackage{pgfplots}
\usetikzlibrary{external}
\tikzset{external/export=false, external/mode=list and make}
\tikzexternalize
\pgfplotsset{compat=1.13}
\begin{document}
    {\tikzset{external/optimize=false}%
    \tikz[remember picture] \node {X};
    }%
    \tikzset{external/export=true}
    \begin{tikzpicture}
        \begin{axis}[clip=false]
            \addplot {x};
            \node (A) at (yticklabel cs:0.5) {A};
        \end{axis};
        \node at (A) {B};
    \end{tikzpicture}
\end{document}