AmCharts - Chat - Changing "valueField"

HTML

<script src="https://www.amcharts.com/lib/3/amcharts.js"></script>
<script src="https://www.amcharts.com/lib/3/serial.js"></script>
<script src="https://www.amcharts.com/lib/3/themes/light.js"></script>
<div id="chartdiv"></div>					
<div id="controls">
    <button onclick="valueField('count')">valueField 'count'</button>
    <button onclick="valueField('ratio')">valueField 'ratio'</button>
</div>

CSS

#chartdiv {
	width	: 100%;
	height	: 500px;
  background-color: white;
}														

#controls {
    margin-top: 20px;
    text-align: center;
}

#controls button {
    width: 200px;
    height: 40px;
}

JavaScript

window.chart = AmCharts.makeChart( "chartdiv", {
  "type": "serial",
  "theme": "light",
  "dataProvider": [ {
    "country": "USA",
    "count": 2025,
    "ratio": 10
  }, {
    "country": "China",
    "count": 1882,
    "ratio": 20
  }, {
    "country": "Japan",
    "count": 1809,
    "ratio": 30
  }, {
    "country": "Germany",
    "count": 1322,
    "ratio": 230
  } ],
  "valueAxes": [ {
    "gridColor": "#FFFFFF",
    "gridAlpha": 0.2,
    "dashLength": 0
  } ],
  "gridAboveGraphs": true,
  "startDuration": 1,
  "graphs": [ {
    "balloonText": "[[category]]: <b>[[value]]</b>",
    "fillAlphas": 0.8,
    "lineAlpha": 0.2,
    "type": "column",
    "valueField": "count"
  } ],
  "chartCursor": {
    "categoryBalloonEnabled": false,
    "cursorAlpha": 0,
    "zoomable": false
  },
  "categoryField": "country",
  "categoryAxis": {
    "gridPosition": "start",
    "gridAlpha": 0,
    "tickPosition": "start",
    "tickLength": 20
  },
  "export": {
    "enabled": true
  }

} );

function valueField(_valueField) {
    window.chart.graphs[0].valueField = _valueField;
		window.chart.validateData();
		window.chart.animateAgain();
}