Drupal: Group more than one fields in a view

template.php

<?php

/**
 * overwrite theme_views_view_list
 *
 * Group more than one fields in a view
 *
 */
function template_views_view_list__VIEWNAME($view, $group, $row) {
  //the field we want to group
  $grouping_fields = array('node_node_data_field_mon_field', 'node_node_data_field_mon_field_2');

  //Arrange in a nice array
  foreach ($view->result as $index => $row) {
    $grouping = array();
    if (isset($view->result[$index]->$grouping_fields[0])) {
      $grouping[0] = $view->result[$index]->$grouping_fields[0];
    }
    if (isset($view->result[$index]->$grouping_fields[1])) {
      $grouping[1] = $view->result[$index]->$grouping_fields[1];
    }
    $sets[$grouping[0]][$grouping[1]][$index] = $grouping[1];
  }

  $sets_sets_sets = $sets;
  $list = array();

  //Go down the array and prepare a list
  foreach ($sets_sets_sets as $main_group => $sets_sets) {
    $group = array();
    foreach ($sets_sets as $sets) {
      $subgroup = array();
      foreach ($sets as $sub_group => $title) {
        //create a row
        $row = '';
        foreach ($view->field as $id => $handler) {
          $row .= '<div>' . $view->style_plugin->rendered_fields[$sub_group][$id] . '</div>';
        }
        //add the row in the group
        $subgroup[] = array('data' => $row);
      }
      $group[] = array($title, 'children' => $subgroup);
    }
    $list[] = array($main_group, 'children' => $group);
  }

  return theme('item_list', $list);
}