Overlays

Overview
A Simple Example
Positioning Overlays Relative to Data

Overview

An overlay is an area laid on top of a Google Chart. It's typically used to call out a particular statistic, but can be anything you want since it's just HTML and CSS.

Simple uses involve creating a CSS class and simply referring to it in your HTML; no JavaScript required. More advanced uses can involve using Google Charts to customize the positioning and content of the overlay.

A Simple Example

For our first example, we'll avoid JavaScript entirely and simply overlay some text on a line chart:

Here, an internal stylesheet defines two classes that we call chartWithOverlay and overlay . Note that we use relative positioning in chartWithOverlay and absolute positioning in overlay .

Then, in the body of our web page, we use a chartWithOverlay as a container into which we place our chart ( line-chart ) and then our overlay .

CSS

  . 
 chartWithOverlay 
  
 { 
 
  
  position 
 : 
  
 relative 
 ; 
 
  
 width 
 : 
  
 700 
 px 
 ; 
  } 
 
  . 
 overlay 
  
 { 
 
  
 width 
 : 
  
 200 
 px 
 ; 
  
 height 
 : 
  
 200 
 px 
 ; 
  
  position 
 : 
  
 absolute 
 ; 
 
  
 top 
 : 
  
 60 
 px 
 ; 
  
 /* chartArea top  */ 
  
 left 
 : 
  
 180 
 px 
 ; 
  
 /* chartArea left */ 
  }

Divs

<div class="chartWithOverlay">

 <div id="line-chart" style="width: 700px; height: 500px;"></div>

 <div class="overlay">
   <div style="font-family:'Arial Black'; font-size: 128px;">88</div>
   <div style="color: #b44; font-family:'Arial Black'; font-size: 32px; letter-spacing: .21em; margin-top:50px; margin-left:5px;">zombie</div>
   <div style="color: #444; font-family:'Arial Black'; font-size: 32px; letter-spacing: .15em; margin-top:15px; margin-left:5px;">attacks</div>
 </div>

</div>

JavaScript

  
< script 
  
 type 
 = 
 "text/javascript" 
  
 src 
 = 
 "https://www.gstatic.com/charts/loader.js" 
>< / 
 script 
>  
< script 
  
 type 
 = 
 "text/javascript" 
>  
 google 
 . 
 charts 
 . 
 load 
 ( 
 "current" 
 , 
  
 { 
 packages 
 : 
 [ 
 'corechart' 
 ]}); 
  
 google 
 . 
 charts 
 . 
 setOnLoadCallback 
 ( 
 drawChart 
 ); 
  
 function 
  
 drawChart 
 () 
  
 { 
  
 var 
  
 data 
  
 = 
  
 new 
  
 google 
 . 
 visualization 
 . 
 arrayToDataTable 
 ([ 
  
 [ 
 'Threat' 
 , 
  
 'Attacks' 
 ], 
  
 [ 
 'Chandrian' 
 , 
  
 38 
 ], 
  
 [ 
 'Ghosts' 
 , 
  
 12 
 ], 
  
 [ 
 'Ghouls' 
 , 
  
 6 
 ], 
  
 [ 
 'UFOs' 
 , 
  
 44 
 ], 
  
 [ 
 'Vampires' 
 , 
  
 28 
 ], 
  
 [ 
 'Zombies' 
 , 
  
 88 
 ] 
  
 ]); 
  
 var 
  
 options 
  
 = 
  
 { 
  
 legend 
 : 
  
 'none' 
 , 
  
 colors 
 : 
  
 [ 
 '#760946' 
 ], 
  
 vAxis 
 : 
  
 { 
  
 gridlines 
 : 
  
 { 
  
 count 
 : 
  
 4 
  
 } 
  
 } 
  
 }; 
  
 var 
  
 chart 
  
 = 
  
 new 
  
 google 
 . 
 visualization 
 . 
 LineChart 
 ( 
 document 
 . 
 getElementById 
 ( 
 'line-chart' 
 )); 
  
 chart 
 . 
 draw 
 ( 
 data 
 , 
  
 options 
 ); 
  
 } 
  
< / 
 script 
>

Full page

<html>
 <head>
   <style> .chartWithOverlay {
           position: relative;
           width: 700px;
    } 
 .overlay {
           width: 200px;
           height: 200px;
           position: absolute;
           top: 60px;   /* chartArea top  */
           left: 180px; /* chartArea left */
    } 
</style>
   <script type="text/javascript" src="https://www.gstatic.com/charts/loader.js"></script>

   <script type="text/javascript">
    google.charts.load("current", {packages:['corechart']});
    google.charts.setOnLoadCallback(drawChart);
    function drawChart() {
      var data = new google.visualization.arrayToDataTable([
        ['Threat', 'Attacks'],
        ['Chandrian', 38],
        ['Ghosts', 12],
        ['Ghouls', 6],
        ['UFOs', 44],
        ['Vampires', 28],
        ['Zombies', 88]
      ]);

      var options = {
        legend: 'none',
        colors: ['#760946'],
        vAxis: { gridlines: { count: 4 } }
      };

      var chart = new google.visualization.LineChart(document.getElementById('line-chart'));
      chart.draw(data, options);
    }
    </script>
 </head>

 <body>
  <div class="chartWithOverlay">

   <div id="line-chart" style="width: 700px; height: 500px;"></div>

   <div class="overlay">
    <div style="font-family:'Arial Black'; font-size: 128px;">88</div>
    <div style="color: #b44; font-family:'Arial Black'; font-size: 32px; letter-spacing: .21em; margin-top:50px; margin-left:5px;">zombie</div>
    <div style="color: #444; font-family:'Arial Black'; font-size: 32px; letter-spacing: .15em; margin-top:15px; margin-left:5px;">attacks</div>
  </div>

 </div>

</body>

</html>

Positioning Overlays Relative to Data

Sometimes the best position for an overlay depends on where the data ends up on the chart. For instance, we might want to place an image close to a data element.

Let's say we wanted to draw attention to the number of zombie attacks in the chart above. We'll do this by placing a scary zombie head at the end of the line.

One way to do this would be to render the chart and hardcode our coordinates. That will work, but would require updating whenever the chart data changes. A more robust solution would have us placing the overlay relative to wherever the data element ends up onscreen. Since we can't know where that will be until the chart has finished rendering, we'll listen for the ready event (called when the chart is done rendering) and access the coordinates programmatically with getXLocation and getYLocation :

CSS

 . 
 chartWithMarkerOverlay 
  
 { 
  
 position 
 : 
  
 relative 
 ; 
  
 width 
 : 
  
 700 
 px 
 ; 
 } 
 . 
 overlay-text 
  
 { 
  
 width 
 : 
  
 200 
 px 
 ; 
  
 height 
 : 
  
 200 
 px 
 ; 
  
 position 
 : 
  
 absolute 
 ; 
  
 top 
 : 
  
 50 
 px 
 ; 
  
 /* chartArea top  */ 
  
 left 
 : 
  
 200 
 px 
 ; 
  
 /* chartArea left */ 
 } 
  . 
 overlay-marker 
  
 { 
  
 width 
 : 
  
 50 
 px 
 ; 
  
 height 
 : 
  
 50 
 px 
 ; 
  
 position 
 : 
  
 absolute 
 ; 
  
 top 
 : 
  
 53 
 px 
 ; 
  
 /* chartArea top */ 
  
 left 
 : 
  
 528 
 px 
 ; 
  
 /* chartArea left */ 
 }