Drawing lines and curves from their central/middle points

Question

I use the following code to draw lines from their central points, to facilitate their positioning in relation to each other.

\documentclass{beamer}
\beamertemplatenavigationsymbolsempty
\usepackage{tikz}
\usepackage{pgfplots}
\begin{document}
\begin{frame}[fragile,t]
\frametitle{}
\begin{tikzpicture}[scale=.9, transform shape]
\draw [thick,-latex](0,0) -- (3,0);
\draw [thick,-latex](0,0) -- (0,4);
\draw [thick, green] (1.8,1.4) node [circle, draw, black, fill=black, scale=0.2]{} +(-50:1.4cm) -- +(130:1.4cm);
\draw [thick, orange] (1.4,1.4) node [circle, draw, black, fill=black, scale=0.2]{} +(50:1.4cm) -- +(-130:1.4cm);
\end{tikzpicture}
\end{frame}
\end{document}

Using the following code, is there a way to draw curves also from their middle points.

\begin{tikzpicture}[scale=.9, transform shape]
\draw [thick,-latex](0,0) -- (4,0);
\draw [thick,-latex](0,0) -- (0,4);
\draw [thick, green] (.88,3.5) to [out=-90,in=180] node [pos=.5,circle, draw, red, fill=red, scale=0.2]{} +(-46.:4.cm);
\draw [thick, blue] (.74,.2) to [out=90,in=-90] node [pos=.5,circle, draw, red, fill=red, scale=0.2]{} +(60:3.8cm);
\end{tikzpicture}

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score 2 · Answer 1 · answered Aug 10 '18 at 05:56

This is certainly not a complete solution. The reason why I do not present a completely automatic solution is that all proposals that I was able to come up with at a given point made use of transform canvas. To give you an example, one thing one could imagine to do is to save the path, look how much off it is and then reuse it with these nice tricks. However, I could only shift the reused paths with transform canvas. While that looks visually OK, ultimately this is still problematic since you could not use the shifted path for intersections.

For all these reasons I would like to argue that on the long run this non-automatic solution might be better. It comes with a style

determine the necessary shift to=<target>

where you feed in your desired center of the curve. If you already sit at the center, the code will remain silent, otherwise it will tell you which xshift and yshift you need to add such that the centers are at the desired coordinates.

\documentclass{beamer}
\beamertemplatenavigationsymbolsempty
\usepackage{tikz}
\usetikzlibrary{decorations.markings,calc}
\tikzset{determine the necessary shift to/.style={save path=\tmpPath,
preaction={decorate,decoration={markings,
mark=at position 0.5 with {\coordinate (aux);
\pgftransformreset
\path let \p1=($#1-(aux)$),\n1={veclen(\x1,\y1)} in \pgfextra{
\pgfmathtruncatemacro{\nTest}{\n1}
\ifnum\nTest=0
\else
\typeout{Please\space add\space xshift=\x1,yshift=\y1\space to your path}
\fi};
};}}}} 

\begin{document}
\begin{frame}[fragile,t]
\frametitle{}
\begin{tikzpicture}[scale=.9, transform shape]
\draw [thick,-latex](0,0) -- (3,0);
\draw [thick,-latex](0,0) -- (0,4);
\draw [thick, green] (1.8,1.4) node [circle, draw, black, fill=black, scale=0.2]{} +(-50:1.4cm) -- +(130:1.4cm);
\draw [thick, orange] (1.4,1.4) node [circle, draw, black, fill=black, scale=0.2]{} +(50:1.4cm) -- +(-130:1.4cm);
\end{tikzpicture}

\begin{tikzpicture}[scale=.9, transform shape]
\draw [thick,-latex](0,0) -- (4,0);
\draw [thick,-latex](0,0) -- (0,4);
\draw [determine the necessary shift to={(1.8,1.4)},green,thick,xshift=8.24234pt,yshift=1.83932pt] (.88,3.5) to [out=-90,in=180]  +(-46.:4.cm);
\draw [determine the necessary shift to={(1.8,1.4)},thick, blue,xshift=7.93524pt,yshift=-7.4265pt] (.74,.2) to [out=90,in=-90] +(60:3.8cm);
\end{tikzpicture}
\end{frame}
\end{document}

Drawing lines and curves from their central/middle points

1 Answers1