如何在jQuery中获得多选的optgroup？

<select id="mySelect" multiple>
  <optgroup label="Group 1">
    <option value="option1">Option 1</option>
    <option value="option2">Option 2</option>
  </optgroup>
  <optgroup label="Group 2">
    <option value="option3">Option 3</option>
    <option value="option4">Option 4</option>
  </optgroup>
</select>

var selectedValues = $('#mySelect').val();

var selectedLabel = $('#mySelect option:selected').parent().attr('label');