No, not in the xml, do this in the recordset.
in the dynamicwebcharts folder, there will be a file named <chartPageName>_data1.php, where <chartPageName> is the name of the page you added th;e chart to. the recordset that creates the chart will be on this page, Double click the recordset to edit it and add to the where clause. Examine how the first parameter was added to the where clause for an example on how to add the second one.