tikz e grafi pesati

Ciao,
sto provando a disegnare grafi con tikz.
Ad esempio:
`
\documentclass{article}

\usepackage{tikz}
\usepackage{verbatim}
\usetikzlibrary{arrows,shapes,positioning}

\begin{document}
\pagestyle{empty}

\tikzstyle{v1}=[circle, draw,inner sep=2pt, minimum width=8pt]
\tikzstyle{edge} = [draw,thick,-,font=\small , double=red, double distance=1pt]
\tikzstyle{edge1} = [draw,thick,-,font=\small]
\tikzstyle{label} = [draw,fill=yellow,font=\normalsize]

\begin{figure}
\begin{tikzpicture}[scale=1, auto,swap]
\foreach \pos/\name in {{(0,0)/a}, {(0,2)/b}, {(1,1)/c}}
\node[v1] (\name) at \pos {};

\draw[edge] (a) to node[label] {+} (b);
\path[edge1] (a) to node[label] {$-$} (c);

\end{tikzpicture}
\end{figure}
\end{document}
`
Vorrei però chè le etichette sugli archi siano al centro dell’arco stesso. Come posso fare?
(per intenderci come qui http://www.texample.net/tikz/examples/bridges-of-konigsberg/)

Togli l’opzione auto.
Bye