\begin{center}
\setlength{\unitlength}{1cm}
\begin{Picture}(-2.5,-2.5)(2.5,2.5)
\cartesiangrid(-2,-2)(2,2)
\polarreference
\pictcolor{blue}
\Put(1,\numberHALFPI){\circle*{0.25}}
\degreesangles
\pictcolor{red}
\Put(1,180){\circle*{0.25}}
\end{Picture}
\end{center}
