vipsoft, this has indeed be one of the most asked questions. If you are keen to start on this, I can help you with dev.

Also, explain how to use the Updates in the plugin (ie. create a Updates/X.php where X is the version of the plugin upgraded to, that can contain php code and sql queries)

It would be very helpful if you could just let us know which hook to use and what array the custom data is stored in. Thanks

You can use "Tracker.Action.record". Here is some code from a plugin we made recently:

function getListHooksRegistered()
{
    return array(
        'Tracker.Action.record' => 'newAction',
    );
}

function newAction($action) {
    try {
        // This fills the $visitor_id variable with Piwik's visitor ID. There are also other variables which you can get access to, just print_r($action);
        $visitor_id = $action->_notificationInfo['idVisit'];
        $data = $action->_notificationObject->getRequest();
        $data = json_decode(stripslashes($data['data']));
    } catch (Exception $e) {
        return false;
    }
    // Can now access custom variables through $data->*, like so:
    $my_custom_variable = $data->my_var;
}

Hope this helps someone.

I'm tempted to mark this as fixed, considering the very nice plugin in: #1436

Agree!

(In [2420]) fixes #1457 - remove E_EXCEPTION (couldn't find any documentation for this as a predefined PHP constant)

refs #998 - merge upgrade.php and common.php so json_decode() is available to Tracker plugins; sync up with -17 release of upgradephp