How to label a TiKz shape for referencing its legend in the caption

Question

How can I label a TiKz line style for further referencing in figure caption. For \addplot the label just follows it. However, I don't know how to do it for \draw.

MWE:

\documentclass{book}
\usepackage{lipsum}
\usepackage{caption}
\usepackage{pgfplots}
\pgfplotsset{compat=1.3}
\tikzset{pointille/.style={dash pattern = on 2pt off 2pt on 6pt off 2pt}}
\tikzset{points/.style={dash pattern = on 1pt off 1pt}}
\tikzset{tirets/.style={dash pattern = on 5pt off 5pt}}

\begin{document}
\lipsum[1]
\begin{figure}[!ht]
\centering
\captionsetup{width=9.5cm}
\pgfplotsset{every axis plot post/.append style={mark=none,line width=1.5pt}}
\begin{tikzpicture}
\begin{axis}[height=7cm,width=9cm,grid=major,xlabel=$x$,ylabel=$f(x)$,
tick label style={font=\footnotesize},label style={font=\small},max space between ticks=45, major tick length=0cm,minor tick length=0cm,enlargelimits=false,]
\addplot[tirets,color=blue]{2*x};\label{p4}
\addplot[pointille,color=green]{0.5*x*x};\label{p5}
\addplot[points,color=red]{-0.125*x*x*x};\label{p6}
\end{axis}

\draw [dashed,red,thick] (1,1) rectangle (2,2); \label{p7}
\draw [solid,blue,thick] (1,1)-- (2,2); \label{p8}
\end{tikzpicture}
\caption[Caption in ToC]{This is a plot about colored curves: $f(x)=2 x$ (\ref{p4}),  $f(x)=0.5 x^2$ (\ref{p5}), and $f(x)=-0.125 x^3$ (\ref{p6}), (\ref{p7}), and (\ref{p8})}
\end{figure}
\lipsum[1]
\end{document}

Answer 1

That \label/\ref functionality is provided by PGFPlots (not TikZ), and only works for plots, not arbitrary \draw commands.

You could achieve the desired result by using an \addplot command for drawing the objects: You can replace \draw [dashed,red,thick] (1,1) rectangle (2,2); with \addplot [dashed,red,thick] coordinates {(1,1)} rectangle (2,2);. You only need to provide the first coordinate of your path in the coordinates list, after that you can use normal TikZ path commands like rectangle or --. Note that the coordinate system is scaled inside the axis environment, so your best bet is to work with (axis cs:<x>,<y>) coordinates instead of trying to find the right values using "normal" coordinates.

\documentclass{book}
\usepackage{lipsum}
\usepackage{caption}
\usepackage{pgfplots}
\pgfplotsset{compat=1.3}
\tikzset{pointille/.style={dash pattern = on 2pt off 2pt on 6pt off 2pt}}
\tikzset{points/.style={dash pattern = on 1pt off 1pt}}
\tikzset{tirets/.style={dash pattern = on 5pt off 5pt}}

\begin{document}
\lipsum[1]
\begin{figure}[!ht]
\centering
\captionsetup{width=9.5cm}
\pgfplotsset{every axis plot post/.append style={mark=none,line width=1.5pt}}
\begin{tikzpicture}
\begin{axis}[height=7cm,width=9cm,grid=major,xlabel=$x$,ylabel=$f(x)$,
tick label style={font=\footnotesize},label style={font=\small},max space between ticks=45, major tick length=0cm,minor tick length=0cm,enlargelimits=false,]
\addplot[tirets,color=blue]{2*x};\label{p4}
\addplot[pointille,color=green]{0.5*x*x};\label{p5}
\addplot[points,color=red]{-0.125*x*x*x};\label{p6}
\addplot [dashed,red,thick] coordinates {(-4,-10)} rectangle (axis cs:-2,-3); \label{p7}
\addplot [solid,blue,thick] coordinates {(-4, -10)} -- (axis cs:-2,-3); \label{p8}
\end{axis}

\end{tikzpicture}
\caption[Caption in ToC]{This is a plot about colored curves: $f(x)=2 x$ (\ref{p4}),  $f(x)=0.5 x^2$ (\ref{p5}), and $f(x)=-0.125 x^3$ (\ref{p6}), (\ref{p7}), and (\ref{p8})}
\end{figure}
\lipsum[1]
\end{document}