This one drove me crazy for awhile too. I managed to solve it by doing the ajax call manually and passing straight json to the chart:
$.ajax({
url: query
}).done(function (results) {
var chart = new FusionCharts({
swfUrl: "/FusionCharts/CandleStick.swf",
width: "100%",
height: '300',
renderAt: this.div,
dataFormat: "json",
dataSource: results
}).render();
}.bind({div: divName})
);