Interacting With the Chart

What we've covered so far is sufficient for many web pages: you've drawn your chart on the page. However, if you want to catch user clicks, or need to manipulate properties or data in a chart that you've already drawn, you need to listen for events thrown by the chart.

All charts throw some kinds of events. Here are the most common:

  • ready - Thrown when the chart is drawn on the page and ready to respond to methods. Listen for this event if you need to request information from the chart.
  • select - Thrown when the user selects something on the chart: typically by clicking on a bar or pie slice.
  • error - Thrown when the chart can't render the data passed in, typically because the DataTable format is wrong.
  • onmouseover and onmouseout - Thrown when the user mouses over or off of a specific chart element, respectively.

Listening for events is simple; simply call google.visualization.events.addListener() passing in a handle to the chart, the name of the event to catch, and the name of a handler to call when the event is thrown. You can call this method with any chart handle, even if you haven't called draw() yet. Note that you can call google.visualization.events.addOneTimeListener() if you want the listener to be called exactly once before removing itself.

Here's a partial code snippet showing how to register to catch a chart's select event:

load libraries...  function drawChart() {    prepare data...    var chart = new google.visualization.PieChart(document.getElementById('chart_div'));    // The select handler. Call the chart's getSelection() method   function selectHandler() {     var selectedItem = chart.getSelection()[0];     if (selectedItem) {       var value = data.getValue(selectedItem.row, selectedItem.column);       alert('The user selected ' + value);     }   }    // Listen for the 'select' event, and call my function selectHandler() when   // the user selects something on the chart.   google.visualization.events.addListener(chart, 'select', selectHandler);    draw the chart...  }

The following shows the Hello Charts code example with a new select event listener. Try it out yourself.

<html>   <head>     <!--Load the AJAX API-->     <script type="text/javascript" src="https://www.gstatic.com/charts/loader.js"></script>     <script type="text/javascript">        // Load the Visualization API and the piechart package.       google.charts.load('current', {'packages':['corechart']});        // Set a callback to run when the Google Visualization API is loaded.       google.charts.setOnLoadCallback(drawChart);        // Callback that creates and populates a data table,       // instantiates the pie chart, passes in the data and       // draws it.       function drawChart() {          // Create the data table.         var data = new google.visualization.DataTable();         data.addColumn('string', 'Topping');         data.addColumn('number', 'Slices');         data.addRows([           ['Mushrooms', 3],           ['Onions', 1],           ['Olives', 1],            ['Zucchini', 1],           ['Pepperoni', 2]         ]);          // Set chart options         var options = {'title':'How Much Pizza I Ate Last Night',                        'width':400,                        'height':300};           // Instantiate and draw our chart, passing in some options.         var chart = new google.visualization.PieChart(document.getElementById('chart_div'));          function selectHandler() {           var selectedItem = chart.getSelection()[0];           if (selectedItem) {             var topping = data.getValue(selectedItem.row, 0);             alert('The user selected ' + topping);           }         }          google.visualization.events.addListener(chart, 'select', selectHandler);             chart.draw(data, options);       }      </script>   </head>   <body>     <!--Div that will hold the pie chart-->     <div id="chart_div" style="width:400; height:300"></div>   </body> </html>

 

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