Piwik.php is not accessed at all


I am trying to implement piwik into a WordPress site but instead of adding the tracking code into the theme I need it to be added in a plugin. Note that if I use the code in the theme it works just fine. When I move it in a plugin (I need this to be able to track other sites using this plugin), the piwik.php file is not accessed at all - nothing in access logs or network traffic in the browser. There are no errors either. Any help or advise would be very helpful.