How to pass value return from API into googlechart (AngularJS)

I want to pass value return from API into google chart, however, I cannot get the value. I got this "Table has no columns."

//map.html

<div ng-if="ctrl.postcode != 'Loading...' && !ctrl.noData" google-chart chart="ctrl.chartObject" style="{{cssStyle}}"></div>

//app.js

function GraphService() {
var chartObject = {
    "type": "LineChart",
    "displayed": true,
    "data": { },
    ....
}

//API

"avgpsf":{"2015-Q2":null,"2015-Q1":null,"2014-Q4":422.69000244141,"2014-Q3":590.6814226423,"2014-Q2":534.81999345259,"2014-Q1":480.51332600911},
"avgpsf2":{"2015-Q2":null,"2015-Q1":null,"2014-Q4":345.67999267578,"2014-Q3":null,"2014-Q2":371.60998535156,"2014-Q1":316.58000183105}

How to pass the value into data in app.js? @_@

Thank you!