var chartdata = [];
while (i < 4 ) {

var chartdataFectecher = DownloadsService.fetchchartdata();
chartdataFectecher.success(function(data){
var chartdata = chartdata.concat(myJsonString);
}
i++;
}

[[{"product_id":"818709874","date":"28 Feb 2014", ...}]",[{"product_id":"818709874","date":"28 Feb 2014" ,..}]"]