segment orientation -1
\documentclass[tikz]{standalone}

\usepackage{pgflibraryshapes}
\usetikzlibrary{backgrounds}
\usetikzlibrary{arrows}
\begin{document}
\begin{tikzpicture}[scale = 1.,font=\fontsize{8}{8}\selectfont]
\path (0.) node(2_0) [draw,shape=circle,color=gray] {2};
\path (2.) node(3_0) [draw,shape=circle,color=gray] {3};
\path (-2.) node(4_0) [draw,shape=circle,color=gray] {4};
\draw[color=gray] (4_0) -- (2_0);
\draw[color=gray] (2_0) -- (3_0);
\path (-1.) node(0_0) [] {};
\path (1.) node(1_0) [] {};
\end{tikzpicture}
\end{document}
segment orientation 0
\documentclass[tikz]{standalone}

\usepackage{pgflibraryshapes}
\usetikzlibrary{backgrounds}
\usetikzlibrary{arrows}
\begin{document}
\begin{tikzpicture}[scale = 1.,font=\fontsize{8}{8}\selectfont]
\path (0.) node(2_0) [draw,shape=circle,color=gray] {2};
\path (-2.) node(3_0) [draw,shape=circle,color=gray] {3};
\path (2.) node(4_0) [draw,shape=circle,color=gray] {4};
\draw[color=gray] (3_0) -- (2_0);
\draw[color=gray] (2_0) -- (4_0);
\path (-1.) node(0_0) [] {};
\path (1.) node(1_0) [] {};
\end{tikzpicture}
\end{document}