Additional attribute for button-Shortcode

Hi,
i need to add additional code for Google Analytics to the a-Tag. Example <a href="http://link-to-somewhere" onclick="_gaq.push(['_link', this.href]); return false;">touch me</a>

So i tried to append some code in the file shortcodes.php. But after that, only a white webpage appears. Here is my extended part of the button code, the Attribute ist named mydat:

function button_shortcode( $atts, $content = null ) {
$defaults = apply_filters( 'arconix_button_shortcode_args', array(
        'size' => 'medium',
        'color' => 'white',
        'url' => '#',
        'target' => '',
        'rel' => ''
	'mydat' => ''
    ) );
	   // Properly escape our data
	$mydat = esc_attr( $mydat );
return "<a href='{$url}' '{$mydat}' class='arconix-button arconix-button-{$size} arconix-button-{$color}'{$rel}{$target}>{$content}</a>";
}

What can i do?

http://ww.wp.xz.cn/plugins/arconix-shortcodes/