With tikz
I can draw a circle in the following way:
\documentclass{minimal}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[x=2cm/2]
\newcommand\rad{1}
\pgfplothandlerlineto
\pgfplotfunction{\x}{-1, -0.99, ..., 1}{\pgfpointxy{\x}{sqrt(\rad*\rad-\x*\x)}}
\pgfplotfunction{\x}{-1, -0.99, ..., 1}{\pgfpointxy{\x}{-sqrt(\rad*\rad-\x*\x)}}
\pgfusepath{stroke}
\end{tikzpicture}
\end{document}
The output is:
I try to do the same thing using parametrisation:
\begin{tikzpicture}[x=2cm/2]
\newcommand\rad{1}
\pgfplothandlerlineto
\pgfplotfunction{\alpha}{0, 0.01, ..., 6.28}{\pgfpointxy{\rad *cos(\alpha)}{\rad *sin(\alpha)}}
\pgfusepath{stroke}
\end{tikzpicture}
How to draw a parametrised curve?