Plot labels all move to the same position

Question

I am attempting to create plot labels whose positions are determined by a function. The root problem is that the labels do not stay where they are placed, but all of the them move to the position of the label last placed.

\documentclass[11pt]{article}
\usepackage{pgfplots}
\usetikzlibrary{calc}

\begin{document}
\def\MyFunction(#1, #2){(#1/1000)^2 * (#2/1000)^2 * 175}

\begin{figure}
\centering
\begin{tikzpicture}
    \begin{axis}
    [
     width=12cm, grid=major,
     ymin=0, ymax=30,
     xmin=0, xmax=1000, domain=0:1000, samples=100
    ]
    \edef\L{450}
    \addplot [solid] {\MyFunction(\L, x)};

    \pgfmathparse{\MyFunction(\L, 500)}
    \edef\tmp{\pgfmathresult}
    \node[anchor=south west, fill=white, fill opacity=0.5] (pt450) at ($(axis cs:500,\tmp)+(0.5cm,-0.5cm)$) {\L 1st};
    \draw[->](pt450.south east)--(pt450.south west)--(axis cs:500,\tmp);

    \edef\L{620}
    \addplot [solid] {\MyFunction(\L, x)};
    \pgfmathparse{\MyFunction(\L, 500)}
    \edef\tmp{\pgfmathresult}
    \node[anchor=south west, fill=white, fill opacity=0.5] (pf620) at ($(axis cs:500,\tmp)+(0.5cm,-0.5cm)$) {\L 2nd};
    \draw[->](pf620.south east)--(pf620.south west)--(axis cs:500,\tmp);
    \end{axis}
\end{tikzpicture}
\end{figure}
\end{document}

My end goal is to use \foreach to have several plots (with labels) generated without the redundancy of code, which is why I am not using different variable names for the second instance. Yes, I will be generating dynamic node names, but removed that here for simplicity.

I'm also open to alternate methods of placing the labels, because this method does seem more complex than it might have to be.

Answer 1

As you said, this problem has to do with expansion issues: PGFPlots delays the actual placement of many objects to a later point (until after the axis limits are definite), and saves the things it needs to place in an unexpanded state. So when your nodes are actually drawn, the macro \L just contains the last value. To work around this, you could use the standard approach of defining a temporary macro in which you expand all the macros whose values you need. In your case, you could use something like this:

\edef\dostuff{%
    \noexpand\node[anchor=south west, fill=white, fill opacity=0.5] (pt450) at ($(axis cs:500,\tmp)+(0.5cm,-0.5cm)$) {\L 1st};
    \noexpand\draw[->](pt450.south east)--(pt450.south west)--(axis cs:500,\tmp);
}\dostuff

However, PGFPlots also provides its own looping command which takes care of expanding the contents of the loop. It's called \pgfplotsinvokeforeach{<list>}{...<code using #1 for the current list element>...}. Using this and the ability of PGFPlots to place nodes along a plot by adding node ... to your \addplot command, your can get this

using the following code

\documentclass[11pt]{article}
\usepackage{pgfplots}
\usetikzlibrary{calc}

\begin{document}
\def\MyFunction(#1, #2){(#1/1000)^2 * (#2/1000)^2 * 175}
\tikzset{
    label node/.style={
        anchor=south west,
        xshift=1em, yshift=-2ex,
        inner sep=2pt,
        pos=0.5,
        name=labelnode,
        append after command={
            (labelnode.south east) -- (labelnode.south west) -- +(-0.8em,1.6ex)}
    }
}


\begin{figure}
\centering
\begin{tikzpicture}
    \begin{axis}
    [
     width=12cm, grid=major,
     ymin=0, ymax=30,
     xmin=0, xmax=1000, domain=0:1000, samples=100
    ]
    \pgfplotsinvokeforeach{420, 620}{
        \addplot [solid] {\MyFunction(#1, x)} node [label node] {#1};
    }
    \end{axis}
\end{tikzpicture}
\end{figure}
\end{document}