Dear Mikael,

An example from PGFPlots Gallery.
Please try if it works. Here on windows, it seems to work.
Attached files are tt.log.txt and tt.pdf.

Best,
Akira

%
% tt.tex: from PGFPlots Gallery
%
\setupcolors[state=start]
\usemodule[tikz]
\usemodule[pgfplots]

\starttext

\midaligned{%
\starttikzpicture
\startaxis[
xmin=-3, xmax=3,
ymin=-3, ymax=3,
extra x ticks={-1,1},
extra y ticks={-2,2},
extra tick style={grid=major},
]
\draw[red] \pgfextra{
\pgfpathellipse{\pgfplotspointaxisxy{0}{0}}
{\pgfplotspointaxisdirectionxy{1}{0}}
{\pgfplotspointaxisdirectionxy{0}{2}}
% see also the documentation of
% 'axis direction cs' which
% allows a simpler way to draw this ellipse
};
\draw[blue] \pgfextra{
\pgfpathellipse{\pgfplotspointaxisxy{0}{0}}
{\pgfplotspointaxisdirectionxy{1}{1}}
{\pgfplotspointaxisdirectionxy{0}{2}}
};
\addplot [only marks,mark=*] coordinates { (0,0) };
\stopaxis
\stoptikzpicture
}

\stoptext