Uncaught ReferenceError: piwikTracker is not defined
So instead I add the dependency like this:
wp_register_script('custom_tracking',get_bloginfo('stylesheet_directory') . '/js/custom_tracking.js', array('jquery', 'wp-piwik') );
Now this works in Chrome but not IE. I can see that the custom_tracking.js is not loaded and the tracking never occurs. If I remove the dependency, the tracking occurs on IE, but shows the error in the console in Chrome.
I posted in the Piwik forums but I don't see why the dependency works in Chrome but not IE and the non-dependency works in IE but not Chrome.