\documentclass{article}
\usepackage{com.braju.graphicalmodels}
\usepackage{amsmath}
% \special{papersize=40mm,80mm}
% \setlength{\pdfpagewidth}{40mm}
% \setlength{\pdfpageheight}{80mm}
% \usepackage[vcentering,dvips]{geometry}
% \geometry{papersize={80mm,120mm},total={80mm,120mm},margin=0mm}



\catcode`\@=11%

\pagestyle{empty}

\begin{document}
\TeXtoEPS

\LARGE
\psset{unit=10mm,arrowscale=1.5,linewidth=1.5pt}

\begin{pspicture}(0,0)(3,4)
  \SpecialCoor

  \rput(1,1){\rnode{x_i}{$x_{i}$}}
  \rput(1,3){\rnode{z_i}{$z_{i}$}}
         
  \psset{arrows=->,nodesep=5pt}
  \ncline{z_i}{x_i}
         
\end{pspicture}

\endTeXtoEPS
\end{document}
