I have the following code, which produces the image below it.
\documentclass[preview]{standalone}
\usepackage{tikz-cd}
\usepackage{pgfplots}
\begin{document}
\begin{figure}[htp]
\centering
\begin{tikzpicture}[scale=1]
\begin{axis}[
xmin=0, xmax=1,
ymin=0, ymax=1,
axis lines=center,
axis on top=true,
axis y line=none,
domain=0:1]
\addplot[only marks]
table {
0.2 0.8
0.8 0.2
};
\addplot[only marks, mark=*,mark options={red}]
table {
0.2 0.2
0.8 0.8
};
\end{axis}
\end{tikzpicture}
\end{figure}
\end{document}
This is working exactly as I want.
But now I want to add a mark right in the middle split between the two colors, half red and half black, more or less like this.
I tried improvising a combination of two halfcircle* options on pgfplots, one red and one black, but they have a white fill, plus an undesired border. This post, from its description, appeared to do something similar to what I want, but for some reason I could not make it work.