Google Chart - Shortcode w/error

function chart_shortcode( $atts ) {
    extract(shortcode_atts(array(
        'data' => '',
        'colors' => '',
        'size' => '400x200',
        'bg' => 'ffffff',
        'title' => '',
        'labels' => '',
        'advanced' => '',
        'type' => 'pie'
    ), $atts));

    switch ($type) {
        case 'line' :
            $charttype = 'lc'; break;
        case 'xyline' :
            $charttype = 'lxy'; break;
        case 'sparkline' :
            $charttype = 'ls'; break;
        case 'meter' :
            $charttype = 'gom'; break;
        case 'scatter' :
            $charttype = 's'; break;
        case 'venn' :
            $charttype = 'v'; break;
        case 'pie' :
            $charttype = 'p3'; break;
        case 'pie2d' :
            $charttype = 'p'; break;
        default :
            $charttype = $type;
        break;
    }

    if ($title) $string .= '&chtt='.$title.'';
    if ($labels) $string .= '&chl='.$labels.'';
    if ($colors) $string .= '&chco='.$colors.'';
    $string .= '&chs='.$size.'';
    $string .= '&chd=t:'.$data.'';
    $string .= '&chf='.$bg.'';

    return '<img title="'.$title.'" src="http://chart.apis.google.com/chart?cht='.$charttype.''.$string.$advanced.'" alt="'.$title.'" />';
}
add_shortcode('chart', 'chart_shortcode');