Provide the argument to use to link from the summary to the next level; this will be called once per row of a summary, and used as part of $view->getUrl().
$data: The query results for the row.
Overrides ArgumentPluginBase::summary_argument
function summary_argument($data) {
// Make sure the argument contains leading zeroes.
return str_pad($data->{$this->base_alias}, 2, '0', STR_PAD_LEFT);
}