I was able to figure this one out. I had to do the following thing in my callback function:
// res is just an xml string that my ajax call returns
var myChart = new FusionCharts("../Charts/Column3D.swf", "RCP6Months", "700", "200", "0", "0", "1");
myChart.setDataXML(res);
myChart.render("myChartDiv");