# Pointing out the running sense on the upper semi-ellipse

I want to add a right arrow somewhere at the middle of the upper semi-ellipse x^2/4+y^2=1,

\documentclass{article}
\usepackage{pgfplots}
\pgfplotsset{compat=1.15}

\begin{document}

\begin{figure}[h]
\begin{tikzpicture}
\begin{axis}
[xlabel=$x$,ylabel=$y$,
xtick={100},ytick={100},
no marks,axis equal,axis lines=middle,
xmin=-2,xmax=2,ymin=-2,ymax=2,
enlargelimits={upper=0.1}]
\addplot[color=black, no markers, samples=1001, samples y=0,
domain=0:pi, variable=\t] ( {(2*cos(\t r)} , {sin(\t r)} );
\end{axis}

\draw (2.98,4.13) node {$1$};
\draw (2.9,2.38) node {$O$};
\draw (5.88,2.42) node {$2$};
\draw (0.3,2.42) node {$-2$};
\end{tikzpicture}
\end{figure}

\end{document}


by to point out the clockwise running sense:

How can I do this ?

• You can use the decorations.markings library as in for example tex.stackexchange.com/questions/370875/add-arrowhead-to-plot/… Unrelated: Instead of adding the nodes with the ticklabels outside the axis environment, place them inside the axis, then you can place them using the coordinates of the axis, e.g. \path node[below left] at (0,0) {$O$} node[above left] at (0,1) {$1$} node[below] at (-2,0) {$-2$} node[below] at (2,0) {$2$}; – Torbjørn T. Dec 13 '17 at 7:56
• @TorbjørnT. Thank you ! I will try to use your suggestion. – Cris Dec 13 '17 at 8:33
• @TorbjørnT. It works well and it's much easier. Unrelated: how could I decrease the length of the ordinate axis Oy, because of the long "tail" of it under the Ox axis ? – Cris Dec 13 '17 at 8:46
• You have the answer to that in your code already (ymin). – Torbjørn T. Dec 13 '17 at 8:48
• I used xmin=-2.5,xmax=2.5,ymin=0,ymax=1, but there is still a long tail under Ox. – Cris Dec 13 '17 at 8:59

You can use decorations.markings to place an arrow inside the path.

\documentclass{article}
\usepackage{pgfplots}
\pgfplotsset{compat=1.15}
\usetikzlibrary{decorations.markings}

\begin{document}

\begin{figure}[h]
\begin{tikzpicture}[
arrow inside/.style = {
postaction={decorate},
decoration={
markings,
mark=at position #1,
}
}]
\begin{axis}
[xlabel=$x$,ylabel=$y$,
xtick={100},ytick={100},
no marks,axis equal,axis lines=middle,
xmin=-2,xmax=2,ymin=-2,ymax=2,
enlargelimits={upper=0.1}]
\addplot[color=black, no markers, samples=1001, samples y=0,
domain=0:pi, variable=\t,arrow inside=.4 with \arrow{<}] ( {(2*cos(\t r)} , {sin(\t r)} );
\end{axis}

\draw (2.98,4.13) node {$1$};
\draw (2.9,2.38) node {$O$};
\draw (5.88,2.42) node {$2$};
\draw (0.3,2.42) node {$-2$};
\end{tikzpicture}
\end{figure}

\end{document}


• @ Henri Menke Thank you for the answer and the full code ! – Cris Dec 13 '17 at 8:32