I have come to rely quite heavily on TikZ, a powerful graphical library for LaTeX.
I write code like
%p% \usetikzlibrary{positioning}
\begin{tikzpicture}[auto]
\node[draw, circle] (s) at (00,15) {$s$};
\node[draw, circle] (a) at (15,15) {$a$};
\node[draw, circle] (b) at (15,00) {$b$};
\node[draw, circle] (t) at (00,00) {$t$};
\path[->] (s) edge node {$1$} (a)
(s) edge node {$2$} (t)
(a) edge node {$1$} (b)
(b) edge node {$1$} (t);
\end{tikzpicture}
in a file, translate it with tikz2png and upload to SE:

TikZ has quite a learning curve but is worth the effort (if you use it to write articles or RPG material, too). Don't try anything without the manual. In case of trouble, tex.SE is by far the best resource, both in terms of available knowledge and responses to new questions.