# Drawing vertical in the middle of a line

I want to draw a vertical in the middle of line. I calculated the coordinate M which represents the middle. Since I know the slope of the original line I want to try to draw the vertical from that M coordinate with tan(-1/m) angle with a specific length. Maybe i misunderstood the example http://www.texample.net/tikz/examples/refraction/ of angles but I guess the second parameter refers to the length (\draw (C3M) -- (-59:8.2cm);). But in my approach this parameter primary effects the angle used. Is maybe the axis environment responsible for this behavior?

\documentclass{minimal}
\usepackage{tikz,pgfplots}
\pgfplotsset{compat=1.11}
\usetikzlibrary{intersections}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
scale only axis, % The height and width argument only apply to the actual axis
height=5cm,
axis x line=middle, axis y line=middle, xlabel={$x_1$},
xlabel style={anchor=north}, ylabel={$x_2$},ylabel style={anchor=south east},
xmin=0, xmax=7.5, ymin=0, ymax=11.5,no marks
]
\addplot[fill=yellow!50,draw=none]coordinates{(0,0)(4,0)(4,3)(2,6)(0,6)};
\addplot[black]coordinates{(0,6)(2,6)};
\addplot[black]{0};
\addplot[black]coordinates{(0,0)(0,6)};
\addplot[black]coordinates{(4,0)(4,3)};
\addplot[black]coordinates{(4,3)(2,6)};
\path (6,0)--coordinate(C3M) node[right]{$3x_1+2x_2\leq 18$} (0,9);
\path (4,0)--coordinate(C1M) node[right]{$x_1\leq 4$} (4,3);
\path (0,6)--coordinate(C2M) node[above]{$x_2 \leq 6$} (2,6);
\draw (C3M) -- (-59:8.2cm);
\draw (C3M) arc (0:-59:2) ;
\end{axis}
\end{tikzpicture}
\end{document}


I wanna end up with something like this:

## 1 Answer

One way is to use partway modifiers

\documentclass{standalone}
\usepackage{pgfplots}
\usetikzlibrary{calc}
\pgfplotsset{compat=newest}
\begin{document}
\begin{tikzpicture}
\begin{axis}[scale only axis, axis height=5cm,
axis x line=middle, axis y line=middle, xlabel={$x_1$},
xlabel style={anchor=north}, ylabel={$x_2$},ylabel style={anchor=south east},
xmin=0, xmax=7.5, ymin=0, ymax=11.5,no marks
]

\addplot[draw=black,fill=yellow!50] coordinates{(0,0) (0,6) (2,6) (4,3) (4,0)} \closedcycle;
\draw[-latex] ($(2,6)!0.5!(4,3)$) coordinate (m) -- ($(m)!3mm!-90:(4,3)$)
node[anchor=30]{$3x_1+2x_2\leq 18$} ;
\end{axis}
\end{tikzpicture}
\end{document}


And avoid using minimal class. It is meant for LaTeX developers. The actual minimal class is article for users.

• 3mm from m in the direction of (4,3) that have been rotated by -90 degree. Feb 11, 2018 at 15:18
• That means perpendicular to (m)--(4,3). Feb 11, 2018 at 15:20