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) {
$value = $this
->caseTransform($data->{$this->base_alias}, $this->options['path_case']);
if (!empty($this->options['transform_dash'])) {
$value = strtr($value, ' ', '-');
}
return $value;
}