pgfplotsinvokeforeach is invoked only twice

Question

I would like to plot the family of curves with equation xy=c for several values of c. I also want each curve to be labeled with the value of c it corresponds to.

Here is my MWE:

\documentclass{standalone}
\usepackage{tikz}
\usepackage{pgfplots}

\begin{document}
\begin{tikzpicture}
    \begin{axis}[
        xmin=-3.0,xmax=3.0,
        ymin=-3.0,ymax=3.0,
        axis x line=middle, 
        axis y line=middle,
        axis equal image,
        cycle list={blue}
        ]
        \pgfplotsinvokeforeach{-3.0,-2.5,...,-0.5,0.5,1.0,...,3.0}{
            \addplot+[domain=-3:-0.1,forget plot] {#1/x};
            \addplot+[domain=0.1:3] {#1/x} node[pos=0.5] {$#1$};
        }
    \end{axis}
\end{tikzpicture}
\end{document}

I get each curve plotted, but only two of them are labeled:

How do I get all twelve curves labeled?

Answer 1

To get finer control over the placement of the nodes, you could adapt my approach from Label plots in pgfplots without entering coordinates manually. For example, you could define a curved path and place all the labels at the intersections of their plot and the path:

\documentclass[border=5mm]{standalone}
\usepackage{tikz}
\usepackage{pgfplots}
\usetikzlibrary{intersections}

\pgfkeys{
    /pgfplots/linelabel/.style={
        name path global=labelpath,
        execute at end plot={
            \path [name path global = labelpositionline]
                (rel axis cs:0.85,0) to [bend left=45, looseness=1] (rel axis cs:0.85,1);
            \path [name intersections={of=labelpath and labelpositionline}] (intersection-1) node [font=\scriptsize] {#1};
        },
    }
}


\begin{document}
\begin{tikzpicture}
    \begin{axis}[
        xmin=-3.0,xmax=3.0,
        ymin=-3.0,ymax=3.0,
        axis x line=middle, 
        axis y line=middle,
        axis equal image,
        cycle list={blue}
        ]
        \pgfplotsinvokeforeach{-3.0,-2.5,...,-0.5,0.5,1.0,...,3.0}{
            \addplot+[domain=-3:-0.1,forget plot] {#1/x};
            \addplot+[linelabel=#1,domain=0.1:3] {#1/x};
        }
    \end{axis}
\end{tikzpicture}
\end{document}

Answer 2

If you use a different value for pos, they will be visible:

\documentclass{standalone}
\usepackage{tikz}
\usepackage{pgfplots}

\begin{document}
\begin{tikzpicture}
    \begin{axis}[
        xmin=-3.0,xmax=3.0,
        ymin=-3.0,ymax=3.0,
        axis x line=middle, 
        axis y line=middle,
        axis equal image,
        cycle list={blue}
        ]
        \pgfplotsinvokeforeach{-3.0,-2.5,...,-0.5,0.5,1.0,...,3.0}{
            \addplot+[domain=-3:-0.1,forget plot] {#1/x};
            \addplot+[domain=0.1:3] {#1/x} node[pos=0.92] {$#1$};
        }
    \end{axis}
\end{tikzpicture}
\end{document}

Answer 3

As the first few commenters pointed out, the labels were placed, just outside the window defined by the pgfplots ymin/ymax keys.

My workaround was to alert the domain of each plotted function so that there were no points outside the window. Then pos=0.5 was exactly where I wanted it.

    \pgfplotsinvokeforeach{-3.0,-2.5,...,-0.5,0.5,1.0,...,3.0}{
        \pgfmathsetmacro{\a}{abs(#1)/3}
        \addplot+[domain=-3:-\a,forget plot] {#1/x};
        \addplot+[domain=\a:3] {#1/x} node[black,fill=white,pos=0.5,sloped,inner sep=0pt] {$#1$};
    }

The disadvantage is that if ymin or ymax were changed the line defining \a would need to be updated. So it's not optimal but the output is not bad.