\documentclass[crop,tikz]{standalone}% 'crop' is the default for v1.0, before
% it was 'preview'
%\usetikzlibrary{...}% tikz package already loaded by 'tikz' option
\begin{document}
\begin{tikzpicture}[xscale=0.8,yscale=1.2]
  \draw (0,4) -- (4,0);
  \draw (2,4) -- (1,3);
  \draw (4,4) -- (5,3);
  \draw (6,4) -- (3,1);
  \draw (4,0) -- (8,4);
  \draw (4,0) -- (4,-1);
  \path[fill=white] (0,4) circle (.3);
  \node at (0,4) {$a$};
  \path[fill=white] (2,4) circle (.3);
  \node at (2,4) {$a$};
  \path[fill=white] (4,4) circle (.3);
  \node at (4,4) {$a$};
  \path[fill=white] (6,4) circle (.3);
  \node at (6,4) {$a$};
  \path[fill=white] (8,4) circle (.3);
  \node at (8,4) {$a$};
  \path[fill=white] (1.5,2) circle (.3);
  \node at (1.5,2) {$x$};
  \node at (4.5,2) {$y$};
  \node at (3.2,.4) {$z$};
  \path[fill=white] (4,-1) circle (.3);
  \node at (4,-1) {$b$};
\end{tikzpicture}
\end{document}
