Why does draw the circle like that? How do I best draw the circle?
\documentclass[border=2pt]{standalone}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
axis equal=true,
axis lines=middle, enlargelimits=false,
xlabel={$x$}, ylabel={$y$},
%
xlabel style={at={(ticklabel* cs:0.983)},anchor=west},
ylabel style={at={(ticklabel* cs:1.03)}, anchor=north west, align=right},
%
tick style={},
%
x tick label style={below},
y tick label style={left},
%
xmin=-10, xmax=10,
ymin=-12, ymax=12,
xtick={}, ytick={},
xticklabels={}, yticklabels={},
%
after end axis/.code={
\path(axis cs:0,0)
node [anchor=north west,yshift=0 cm, xshift=0 cm] {}
node [anchor=north east,xshift=0 cm] {O};}]
%
\addplot[domain=-5:8,very thick,red,smooth] {4-x};
\addplot [domain=-10:10,,very thick, blue,samples=50,smooth] {4+sqrt(32-x^2)};
\addplot [domain=-10:10,,very thick, blue,samples=50,smooth] {4-sqrt(32-x^2)};
\end{axis}
\end{tikzpicture}
\end{document}