centre text exactly on an exactly specified point on path

Question

I would like to place text exactly centred over an exactly specified point on a highly complex path as shown in this Figure:

I have developed my example by specifying the point at 0.1 from the beginning of path and adding a mark to it:

\documentclass[border=1cm]{standalone}

\usepackage{tikz}
\usetikzlibrary{decorations.text, decorations.markings}

\begin{document}
\begin{tikzpicture}

\coordinate (A) at (0,0);
\coordinate (B) at (1, 3);
\coordinate (C) at (2, -1);
\coordinate (D) at (3, 0.5);

%placement of mark                                                                                                                                                                                          
\draw plot [smooth] coordinates { (A) (B) (C) (D) };
\path [
   postaction={decorate},
     decoration={
       markings,
       mark = at position 0.1 with {\fill[red] circle[radius=0.2cm];}
  }
] plot [smooth] coordinates { (A) (B) (C) (D) };

%placement of text                                                                                                                                                                                          
\path [
   postaction={decorate},
     decoration={
       text along path,
       raise=0.4cm,
       text align/left indent = {0.1\dimexpr\pgfdecoratedpathlength\relax},
       text={here!!}
  }
] plot [smooth] coordinates { (A) (B) (C) (D) };

\end{tikzpicture}
\end{document}

The output is

To centre the text over the mark I would only need to replace left with center in text align/left indent = {0.1\dimexpr\pgfdecoratedpathlength\relax}. Then everything would be perfect! Unfortunately after this change the example no longer compiles.

How to fix this problem?

Answer 1

Just add the text to the markings. (You need transform shape to rotate it to become typeset along the tangent at that point.)

\documentclass[border=1cm]{standalone}

\usepackage{tikz}
\usetikzlibrary{decorations.markings}

\begin{document}
\begin{tikzpicture}

\coordinate (A) at (0,0);
\coordinate (B) at (1, 3);
\coordinate (C) at (2, -1);
\coordinate (D) at (3, 0.5);

%placement of mark                                                                                                                                                                                          
\draw plot [smooth] coordinates { (A) (B) (C) (D) };
\path [
   postaction={decorate},
     decoration={
       markings,
       mark = at position 0.1 with {\fill[red] circle[radius=0.2cm];
       \node[transform shape] at (0,0.5) {here!};}
  }
] plot [smooth] coordinates { (A) (B) (C) (D) };

%placement of text                                                                                                                                                                                          
\end{tikzpicture}
\end{document}

Alternatively you can just subtract 50% of the width from the shift. (Note that I use in this code \pgfmathsetmacro{\mywidth}{5*width("here!!")} because 5*0.1=1/2 so in 0.1\dimexpr\pgfdecoratedpathlength-\mywidth pt\relax we subtract 50% of the width of the text.)

\documentclass[border=1cm]{standalone}

\usepackage{tikz}
\usetikzlibrary{decorations.text, decorations.markings}

\begin{document}
\begin{tikzpicture}

\coordinate (A) at (0,0);
\coordinate (B) at (1, 3);
\coordinate (C) at (2, -1);
\coordinate (D) at (3, 0.5);

%placement of mark                                                                                                                                                                                          
\draw plot [smooth] coordinates { (A) (B) (C) (D) };
\path [
   postaction={decorate},
     decoration={
       markings,
       mark = at position 0.1 with {\fill[red] circle[radius=0.2cm];}
  }
] plot [smooth] coordinates { (A) (B) (C) (D) };

\pgfmathsetmacro{\mywidth}{5*width("here!!")}
%placement of text                                                                                                                                                                                          
\path [
   postaction={decorate},
     decoration={
       text along path,
       raise=0.4cm,
       text align/left indent ={0.1\dimexpr\pgfdecoratedpathlength-\mywidth pt\relax},
       text={here!!}
  }
] plot [smooth] coordinates { (A) (B) (C) (D) };

\end{tikzpicture}
\end{document}

Some style for that. (Note that I use the hobby path simply because otherwise there can be dimension too large problems. These problems have nothing to do with the specifics of the style, it is a general problem of decorated paths.)

\documentclass[tikz,border=1cm]{standalone}
\usetikzlibrary{decorations.text, decorations.markings,hobby}
\tikzset{move text/.style args={#1 to pos #2}{%
postaction={decorate,
/utils/exec=\pgfmathsetmacro{\mywidth}{(0.5/#2)*width("#1")},
     decoration={
       text along path,
       raise=0.4cm,
       text align/left indent={#2\dimexpr\pgfdecoratedpathlength-\mywidth pt\relax},
       text={here!!}
  }}
}}
\begin{document}
\foreach \X in {0.1,0.2,...,0.9}
{\begin{tikzpicture}[]
\path[use as bounding box] (-2,-1) rectangle (4,4);
\coordinate (A) at (0,0);
\coordinate (B) at (1, 3);
\coordinate (C) at (2, -1);
\coordinate (D) at (3, 0.5);

\draw [
   postaction={decorate,
     decoration={
       markings,
       mark = at position \X\space with {\fill[red] circle[radius=0.2cm];}
  }},move text={here!!} to pos \X,
] plot [smooth,hobby] coordinates { (A) (B) (C) (D) };

\end{tikzpicture}}
\end{document}