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I need a picture like

The graph

Here is a try:

\documentclass{article}
\usepackage{pgfplots}
\usetikzlibrary{decorations.markings}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}
\draw[-stealth,  thick](0,0)--(4,0) node[below]{$x$};
\draw[-stealth, thick](2,-2)--(2,2) node[left]{$y$};

\draw (2,0) circle (1.2cm);

\end{tikzpicture}
\end{document}
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1  
You probably need to read the manual, the first chapter is excellent for you. Consider how you placed x and y. In exactly the same way can you place a and b, depending on how you wish to do it. Welcome to TeX.SE! :) –  zeroth Jan 23 at 7:53
2  
Something like \draw [fill=gray] (2.5,0.5) circle (2pt) node [above left] {$a$}; should be what you are looking for. –  Peter Grill Jan 23 at 8:07
    
Thanks! It works perfectly –  Leo Jan 23 at 8:11
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4 Answers 4

up vote 4 down vote accepted

A PSTricks solution:

\documentclass{article}

\usepackage{pstricks-add}

\begin{document}

\begin{pspicture}(-3.9,-3.9)(4.4,4.4)
  \psaxes[labels = none, ticks = none]{->}(0,0)(-3.9,-3.9)(4,4)[$x$,0][$y$,90]
  \pscircle(0,0){3}
  \pnodes(1.8,1.2){A}(3.3,3.3){B}
  \psdots(A)(B)
  \uput[135](A){a}
  \uput[135](B){b}
\end{pspicture}

\end{document}

output

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Just to add a tikz solution to a tikz question. You can draw nodes:

\documentclass{article}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}
\draw[-stealth,  thick](0,0)--(4,0) node[below]{$x$};
\draw[-stealth, thick](2,-2)--(2,2) node[left]{$y$};

\draw (2,0) circle (1.2cm);

\node[circle,fill,label={120:a},inner sep=1.5pt,outer sep=-2pt] at (2.8,0.5) {};
\node[circle,fill,label={120:b},inner sep=1.5pt,outer sep=-2pt] at (3.2,1) {};

\end{tikzpicture}
\end{document}

enter image description here

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With PGFPlots:

\documentclass{article}

\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture} 
\begin{axis}[
    axis equal image,
    axis lines=middle,
    enlargelimits=0.2,
    xtick=\empty, ytick=\empty,
    xlabel=$x$, ylabel=$y$
]
\addplot [thick, smooth, domain=0:360, data cs=polar] (x,1);
\addplot [only marks, black, mark=*, nodes near coords, point meta=explicit symbolic] table [meta=label] {
x   y   label
0.5 0.4 a
0.8 1   b
};
\end{axis}
\end{tikzpicture}

\end{document}
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Another solution with PSTricks just for fun! It might be simpler than the existing one.

\documentclass[pstricks,border=12pt,12pt]{standalone}
\usepackage{pst-eucl,pst-plot}

\begin{document}

\begin{pspicture}(-4,-4)(4.5,4.5)
  \psaxes[labels=none,ticks=none]{->}(0,0)(-4,-4)(4,4)[$x$,0][$y$,90]
  \pscircle{3}
  \pstGeonode[PosAngle=120](2,1){a}(3.5,3.5){b}
\end{pspicture}

\end{document}

enter image description here

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