@DavidCarlisle didn't you recently said: use picture mode as this doesn't have bugs? Try this: \documentclass{article}
\begin{document}
\setlength{\unitlength}{1cm}
\begin{center}
\begin{picture}(10,10)
\put(5,5){\line(2,-2){2}}
\end{picture}
\end{center}
\end{document}