This event is signaled by _gather() in lib/plugins/popularity/helper.php when the popularity plugin gathers the data to send, handlers can use it to add more data.
The passed Doku_Event object has the field: $data.
$data field is a key-value array passed by reference, in which plugins can add data. The key inserted should be the name of the plugin. The value inserted should be either a string, or a key-value array.
The BEFORE and AFTER events let you add entries to the $data array.