Returns all field lables of fields before this field.
array An array of field labels keyed by their field IDs.
protected function getPreviousFieldLabels() {
$all_fields = $this->view->display_handler
->getFieldLabels();
$field_options = array_slice($all_fields, 0, array_search($this->options['id'], array_keys($all_fields)));
return $field_options;
}