Node in a middle of a line strange behaviour

Question

As you can see, the Greek letter theta which is expected to be just below the curved stroke instead takes on a completely unexpected position. What could it be due to?

\documentclass[14pt]{article}

\usepackage{tikz}
\usetikzlibrary{arrows}
\usetikzlibrary{snakes}
\usetikzlibrary{decorations.markings}
\usetikzlibrary{shapes.arrows}
\usepackage{xcolor}
\definecolor{ocra}{rgb}{0.72,0.53,0.04}
\definecolor{verde}{HTML}{00C200}
\begin{document}

\begin{figure}
\centering
\begin{tikzpicture}[thick]
   \draw[gray,snake=coil,segment length=3pt] (-0.5,3.5) -- (0.75,5) node [midway, right,yshift=3px] (sx) {};
   \draw[gray,snake=coil,segment length=3pt] (0.75,5)-- (2,3.5)     node [midway, left,yshift=3px]  (dx){};
   \node[circle,draw=blue,fill=blue!25,scale=0.75,minimum size=0.1cm] at (-0.5,3.5) (A) {\textcolor{blue}{A}};
   \node[circle,draw=red,fill=red!25,scale=0.75,minimum size=0.1cm] at (0.75,5) (B) {\textcolor{red}{B}};
   \node[circle,draw=ocra,fill=ocra!25,scale=0.75,minimum size=0.1cm] at (2,3.5) (C) {\textcolor{ocra}{C}};
   
   \node[xshift=0.5cm,yshift=-0.5cm] at (A) (a) {};
   \draw[->] (a) -- ++ (-1,0);
   
   \node[xshift=-0.5cm,yshift=-0.5cm] at (C) (c) {};
   \draw[->] (c) -- ++ (1,0);
   \draw[] (sx) to [in=255,out=-75] (dx) node [midway,below] (theta) {$\theta$};;
   
   \end{tikzpicture}
\end{figure}
\end{document}

Answer 1

What you are encountering is the behaviour of nodes with to paths. A to path is a special path that might be built out of quite complicated instructions. When TikZ encounters a to path, therefore, it packages up various pieces of information to pass off to the construction code. In particular, it records the starting coordinate, the target (end) coordinate, and any nodes that are meant to be placed along the curve. The to path then constructs the curve and hands control back to TikZ.

Key to this is how TikZ knows which nodes should lie along the curve. They have to be defined before the target coordinate. Any nodes after that are constructed outside the to curve and so aren't privy to any information relating to that curve. This becomes a problem when using positioning keys such as midway (which is an alias for pos=.5) since there is not a path that this is "midway" along. The node's position then defaults to (0,0) which, because of how your diagram is constructed, is a long way from the diagram itself.

So to fix your issue, you need to put the theta node declaration before the target coordinate, (dx).

Here's a simplified version showing the problem and its solution.

\documentclass{article}
%\url{https://tex.stackexchange.com/q/718033/86}

\usepackage{tikz}
\begin{document}

\begin{tikzpicture}
\draw (0,0) circle[radius=3mm];
\draw (0,2) to node[midway] {A} +(1,0) ;
\draw (0,1) to +(1,0) node[midway] {B} ;
   
\end{tikzpicture}

\end{document}

Answer 2

I've simplified the code by creating a style my node and using the angles library. I've eliminated superfluous code by using only A, B and C nodes.

\documentclass[14pt]{article}

\usepackage{tikz}
\usetikzlibrary{arrows,snakes,decorations.markings}
\usetikzlibrary{shapes.arrows,angles}
\usepackage{xcolor}
\definecolor{ocra}{rgb}{0.72,0.53,0.04}
\definecolor{verde}{HTML}{00C200}

\tikzset{my node/.style={circle,draw=#1,fill=#1!25,scale=0.75,minimum size=0.1cm,text=#1},
decoration={snake=coil,segment length=3pt}}

\begin{document}

\begin{figure}
\centering
\begin{tikzpicture}[thick]
   \node[my node=blue] at (-0.5,3.5) (A) {A};
   \node[my node=red]  at (0.75,5) (B) {B};
   \node[my node=ocra]  at (2,3.5) (C) {C};
   \draw[gray,decorate] (A)--(B)--(C); 
   \pic [draw,angle radius=1cm,shorten >=1pt,shorten <=3pt,pic text=$\theta$,angle eccentricity=1.2] {angle = A--B--C};
   \draw[->] (A) ++ (.5,-.75)--++(-1,0);
   \draw[->] (C) ++ (-.5,-.75)--++(1,0);
   
   \end{tikzpicture}
\end{figure}
\end{document}

Answer 3

The unexpected placement of the Greek letter theta may be attributed to its positioning as a node midway between two points along the snake curve. To resolve this issue, setting pos=0.5 will precisely position it at the midpoint of the curve.

\documentclass[14pt]{article}
\usepackage{tikz}
\usetikzlibrary{arrows}
\usetikzlibrary{decorations.pathmorphing}
\usetikzlibrary{decorations.markings}
\usetikzlibrary{shapes.arrows}
\usepackage{xcolor}
\definecolor{ocra}{rgb}{0.72,0.53,0.04}
\definecolor{verde}{HTML}{00C200}
\begin{document}

\begin{figure}
\centering
\begin{tikzpicture}[thick]
   \draw[gray,decorate,decoration={coil,segment length=3pt}] (-0.5,3.5) -- (0.75,5) node [midway, right,yshift=3pt] (sx) {};
   \draw[gray,decorate,decoration={coil,segment length=3pt}] (0.75,5)-- (2,3.5)     node [midway, left,yshift=3pt]  (dx){};
   \node[circle,draw=blue,fill=blue!25,scale=0.75,minimum size=0.1cm] at (-0.5,3.5) (A) {\textcolor{blue}{A}};
   \node[circle,draw=red,fill=red!25,scale=0.75,minimum size=0.1cm] at (0.75,5) (B) {\textcolor{red}{B}};
   \node[circle,draw=ocra,fill=ocra!25,scale=0.75,minimum size=0.1cm] at (2,3.5) (C) {\textcolor{ocra}{C}};
   
   \node[xshift=0.5cm,yshift=-0.5cm] at (A) (a) {};
   \draw[->] (a) -- ++ (-1,0);
   
   \node[xshift=-0.5cm,yshift=-0.5cm] at (C) (c) {};
   \draw[->] (c) -- ++ (1,0);
   \draw[] (sx) to [in=255,out=-75] node [pos=0.5, below] (theta) {$\theta$} (dx);
    \end{tikzpicture}
\end{figure}
\end{document}

Answer 4

With use of recent tikz syntax, using tikz libraries angles, arrows.meta, decorations.pathmorphing, quotes and positioning, defining all picture elements styles as the tikzpicture options the MWE, which produce desired result can be:

\documentclass[margin=3pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{angles, arrows.meta,
                decorations.pathmorphing,
                quotes,
                positioning}

\begin{document}
    \begin{tikzpicture}[
node distance = 21mm and 7mm,
            > = {Straight Barb[scale=0.8]},
     C/.style = {% C as Circle   
                 circle, draw=#1,fill=#1!30, text=#1,
                 minimum size=1.1em, inner sep=0pt},
     MA/.style = {% MA as My Angle
                  draw, <->, font=\footnotesize,
                  angle radius=11mm,  shorten >=3pt,shorten <=3pt,
                  angle eccentricity=1.2},     
     S/.style = {draw=gray, thick, decorate,
                 decoration={snake, amplitude=2pt, 
                            segment length=4pt, pre length=1pt, post length=1pt},
                },
                        ]
\node (a) [C=red]   {A};
\node (b) [C=blue, below  left=of a] {B};
\node (c) [C=ocra, below right=of a] {C};
%
\draw [S]   (a) -- (b);
\draw [S]   (a) -- (c);
%
\pic [MA, "$\theta$"] {angle = b--a--c};
%
\draw[->] ([sy=-4] b.east) --  ([sy=-4] b.west);
\draw[->] ([sy=-4] c.west) --  ([sy=-4] c.east);
    \end{tikzpicture}
\end{document}

Addendum:
I read your question again and now I'm not sure what is your problem actually. Hence I edited a bit original answer and add a new answer for another possible solution:

\documentclass[margin=3pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{angles, arrows.meta,
                decorations.pathmorphing,
                quotes,
                positioning}
\definecolor{ocra}{rgb}{0.72,0.53,0.04}

\begin{document}
    \begin{tikzpicture}[
node distance = 21mm and 7mm,
            > = {Straight Barb[scale=0.8]},
     C/.style = {% C as Circle   
                 circle, draw=#1,fill=#1!30, text=#1,
                 minimum size=1.1em, inner sep=0pt},
    MA/.style = {% MA as My Angle
                  draw, <->, font=\footnotesize,
                  angle radius=11mm,  shorten >=3pt, shorten <=3pt,
                  angle eccentricity=1.2},     
    sy/.style = {yshift=#1mm},
     S/.style = {draw=gray, thick, decorate,
                  decoration={snake, amplitude=2pt, 
                              segment length=4pt, pre length=1pt, post length=1pt},
                },
every edge quotes/.style = {fill=white,font=\footnotesize}]
                        ]
\node (a) [C=red]   {A};
\node (b) [C=blue, below  left=of a] {B};
\node (c) [C=ocra, below right=of a] {C};
%
\draw [S]   (a) -- (b);
\draw [S]   (a) -- (c);
%
\pic [MA, "$\theta$"] {angle = b--a--c};
%
\draw[<->] ([sy=-2] b.south) to ["$\vartheta$"]  ([sy=-2] c.south);
    \end{tikzpicture}
\end{document}