var my_div = document.getElementById('chart_div');
 var chart = new google.visualization.ChartType(chart_div);

 google.visualization.events.addListener(my_chart, 'ready', function () {
    var imgUri = chart.getImageURI();
    //perform print function
 });

chartEditor = new google.visualization.ChartEditor();

   google.visualization.events.addListener(wrapper, 'ready', function () {
      var imgUri = chartEditor.getChart().getImageURI();
      //throws error: getChart().getImageURI() not a function of chartEditor
   });

chartEditor.getChartWrapper().getChart().getImageURI();