protected function writeGraphHeader()
    {
        $nodeAttributes = array(
            'fontname'  => "Verdana",
            'fontsize'  => 8,
            'shape'     => "none",
            'margin'    => 0,
            'fillcolor' => '#FEFECE',
            'style'     => 'filled',
        );

        $edgeAttributes = array(
            'fontname'  => "Verdana",
            'fontsize'  => 8,
        );

        $indent = 1;

        $graph  = $this->formatLine('digraph ' . $this->graphId . ' {');
        $graph .= $this->formatLine('overlap = false;', $indent);
        $graph .= $this->formatLine('node [' . $this->attributes($nodeAttributes) . '];', $indent);
        $graph .= $this->formatLine('edge [' . $this->attributes($edgeAttributes) . '];', $indent);

        return $graph;
    }