How can I draw a full circle at the origin when I have the y-axis to the left and the x-axis in the bottom?

Question

I am trying to draw a circular node at the origin,while keeping the y-axis to the left and the x-axis to the bottom. The node results to be a quarter of a circle. How can I keep it full circle while preserving the axis?

Below is the code I am using.

\begin{tikzpicture}[baseline]
\begin{axis}[
    axis y line = left,
    axis x line=bottom,
    xlabel = t,
    xtick = {\empty},
    ytick =  {\empty},
]
\addplot [
    domain=-6:6, 
    samples=100, 
    color=gray,
    line width = 0.55mm,
    ]
    {1/(1+exp(-\x))};
   \draw[fill] (0,0) circle[radius=3pt];  
    \end{axis} ;

\end{tikzpicture}
}

Thank you!

Answer 1

One possibility is

after end axis/.code={\draw[fill] (current axis.origin) circle[radius=3pt];}

to avoid that the node is clipped at the border of the axis. Instead (current axis.origin) you could also use (axis cs:0,0) in the code above.

\documentclass[margin=5pt]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\begin{document}
\begin{tikzpicture}[baseline]
\begin{axis}[
    axis y line = left,
    axis x line=bottom,
    xlabel = t,
    xtick = {\empty},
    ytick =  {\empty},
    after end axis/.code={\draw[fill] (current axis.origin) circle[radius=3pt];}
]
\addplot [
    domain=-6:6, 
    samples=100, 
    color=gray,
    line width = 0.55mm
    ]
    {1/(1+exp(-\x))};
    \end{axis}
\end{tikzpicture}
\end{document}

Answer 2

I'd use

\addplot[color=black,only marks,mark=*] (-6,0);

which gives

here's a complete MWE:

% arara: pdflatex
\documentclass{standalone}
\usepackage{pgfplots}

\begin{document}

\begin{tikzpicture}
\begin{axis}[
    axis y line = left,
    axis x line=bottom,
    xlabel = t,
    xtick = {\empty},
    ytick =  {\empty},
]
\addplot [
    domain=-6:6, 
    samples=100, 
    color=gray,
    line width = 0.55mm,
    ]
    {1/(1+exp(-\x))};
   \addplot[color=black,only marks,mark=*] (-6,0);
    \end{axis} 

\end{tikzpicture}


\end{document}