Define node at relative coordinates of \draw plot

Question

I am not able to fix that simple plot. I think I missed something, but I can't point out what ! Could you tell me why (a) is not at midway of the red line ?

\documentclass{article}

\usepackage{tikz}

\begin{document}

\begin{tikzpicture}[domain=0:4]
\draw[very thin,color=gray] (-0.1,-1.1) grid (3.9,3.9);
\draw[->] (-0.2,0) -- (4.2,0) node[right] {$x$};
\draw[->] (0,-1.2) -- (0,4.2) node[above] {$f(x)$};
\draw[color=red] plot (\x,\x) node [midway,inner sep=0] (a) {} node[right] {$f(x) =x$};
\draw[->] (4,1) node[right] {this is not required (a)} -- (a) ;
\node[circle,fill,label={above left:this should be (a)}] at (2,2) {};

\end{tikzpicture}

\end{document}

Is this a duplicate of tikz-parabola-node-position ?

Answer 1

Unfortunately, the midway (or pos=0.5) syntax doesn't work for TikZ plots (it would work if you used pgfplots to draw your plots).

However, you can use the decorations.markings library to define a coordinate along the path. Here's a style mark position=<pos>(<coordinate name>) that places a coordinate with a specified name at a specified distance along the path. You can call this multiple times if you need more than one position on a path.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}
\begin{document}


\tikzset{
    mark position/.style args={#1(#2)}{
        postaction={
            decorate,
            decoration={
                markings,
                mark=at position #1 with \coordinate (#2);
            }
        }
    }
}

\begin{tikzpicture}[domain=0:4]
\draw[very thin,color=gray] (-0.1,-1.1) grid (3.9,3.9);
\draw[->] (-0.2,0) -- (4.2,0) node[right] {$x$};
\draw[->] (0,-1.2) -- (0,4.2) node[above] {$f(x)$};
\draw[color=red, mark position=0.5(a)] plot (\x,\x) node[right] {$f(x) =x$};
\node at (a) [left] {this is (a)};



\end{tikzpicture}

\end{document}