\documentclass{standalone}

\usepackage{pgfplots}

\pgfplotsset{compat=1.8}

\usepgfplotslibrary{ternary}

\begin{document}

\begin{tikzpicture}

\begin{ternaryaxis}[
ternary limits relative=false,
width=7.5cm,
height=7.5cm,
ymax=0.8,
clip=false,
%disabledatascaling,
]

\addplot3[only marks, mark = x]
table{
0    0.8    0
};

\fill[red,opacity=0.5] (axis cs:0,0.8,0) circle(2pt);

\fill[red,opacity=0.5] (axis cs:0,0.8) circle(2pt);

\end{ternaryaxis}

\end{tikzpicture}

\end{document}
