capture image on google chart

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ASP.NET

Javascript

jQuery

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Hi,
Please help me on this.. i am currently using the google chart API, all is running well
but then a specs came that the image should be captured,,on this i called the getImageURI()
function of the google chart API,, Calling this function after the chart is ready,, this came from the google : Note: You should wait for the ready event, as shown in the examples on this page, to ensure that the chart has finished rendering.

var my_div = document.getElementById('my_div');
    var my_chart = new google.visualization.ChartType(chart_div);
    var imgUri;
      google.visualization.events.addListener(my_chart, 'ready', function () {
      imgUri = chart.getImageURI();

      document.getElementById("<%= HiddenField1.ClientID%>").value = imgUri;
      });

    my_chart.draw(data);

My HiddenField1 will be populated after the chart is already rendered..or simply speaking when the page is already done loading. this happens when i clicked a preview button.

However my specs indicated that HiddenField1 should be populated right after the print button is clicked, means it should have a value right before it finish rendering the asp page..

Ex current scenario :

Step 1: button preview is clicked --> the hidden field is populated with png string value.
Step 2: button print is clicked --> the hidden field has value so i can print png image.

Expected scenario :
button print is clicked --> on fly right before the page finish loading or the chart is ready, hidden field should already has a value so that i can print a png image.

I hope you can input on this, i read some of the articles on setTimeOut function on javascript, but i can apply it in my code.

Thanks,!
Jayson