It would be really nice if you could customize the tooltips. Documentation

Some thoughts for a starting point:
Actually the data-Array is created by google.visualization.arrayToDataTable(chart.dataArray) which makes it impossible. So it will be necessary to loop through the data array and add a new row for each value instead. If custom tooltips are given then you need to add an additional column with role: 'tooltip'.

On PHP side the tooltips could be provided by an (optional) tooltip array within the $options passed to the theme function (and then passed to Drupal.settings).

Example for the $options array:

$options = array(
    'title' => 'Sales',
    'fields' => array(
      'date' => array(
        'label' => t('Date'),
      ),
      'count' => array(
        'enabled' => TRUE,
        'label' => t('Sold products'),
      ),
    ),
    'xAxis' => array(
      'labelField' => 'date',
    ),
    'type' => 'line',
    'data' => array(
      array(
        'count' => 5,
        'date' => 'January',
      ),
      array(
        'count' => 11,
        'date' => 'February',
      ),
    ),
    'tooltips' => array('5 sales in jan', '11 sales in feb'),
  );