$sql = mysqli_query($link, "SELECT parameter_id, min_value, number_hit FROM intervals");
if(!$sql) {
  print_r(mysqli_error($link));
}

while($row = mysqli_fetch_assoc($sql)) {
  if($crowd[$row['parameter_id']] == null) {
    $crowd[$row['parameter_id']] = array('interval' => array(), 'hits' => array());
  }

  // try outputting: $crowd[$row['parameter_id']]

  // try outputting: round($row['min_value'],2);
  // try outputting: $row['number_hit'];


  $crowd[$row['parameter_id']]['interval'][] =  round($row['min_value'],2);
  $crowd[$row['parameter_id']]['hits'][] = $row['number_hit'];

  // try outputting: $crowd[$row['parameter_id']]
}