Charts : search-fields limit


There is a limit of 2 fields in the search-fields definition for charts

<chart name="" title="foo">
      <field name="f1" title="Field 1" type="datetime"/>
      <field name="f2" title="Field 2" type="datetime"/>
    <dataset type="sql">...</dataset>

Is there a way to bypass this limitation ? Or can I create a form and tie its values to my chart ?