save google gauge chart as a png - charts

Question

I am using google charts and have a page of mixed charts, some pie, a column chart and a gauge chart
The page has an option to generate a pdf, so I am converting the charts to PNG to use in the pdf..
all the charts are generated in the same manner, using a div to display the google chart and a hidden div to store the png image
var chart = new google.visualization.Gauge(document.getElementById('gauge'));
var hidden = new google.visualization.Gauge(document.getElementById('gauge_hidden'));
// Wait for the chart to finish drawing before calling the getImageURI() method.
google.visualization.events.addListener(chart, 'ready', function () {
gauge_hidden.innerHTML = '<img src="' + chart.getImageURI() + '">';
});
chart.draw(data, options);
this code works fin on he pie and column charts, but on the gauge chart I am seeing
chart.getImageURI is not a function
any ideas how I can get the png?
CHeers

Answer 1

I was facing this same issue, but after some reading i've come to this solution:
I'm using jQuery to make this a little easier.
First, use XMLSerializer to convert the SVG chart to a string and then use btoa to convert that to base64.
You can use this string this way:
var s = new XMLSerializer().serializeToString($(chart_div).find('svg')[0]);
var base64String = "data:image/svg+xml;base64," + window.btoa(s);
In my case i needed to draw the gauge chart to a PDF and DOMPDF doesn't support this format, so if you need a "data:image/png;base64," string, you can continue with this solution.
You need to set the "svg+xml;base64" as src of a new Image and then draw that image to a Canvas. After that you can use toDataURL method from canvas to get the content as base64 png.
var s = new XMLSerializer().serializeToString($(chart_div).find('svg')[0]);
var image = new Image();
image.width = 640;
image.height = 480;
image.src = 'data:image/svg+xml;base64,' + window.btoa(s);
var myCanvas = document.createElement('canvas');
myCanvas.width = 640;
myCanvas.height = 480;
var myCanvasContext = myCanvas.getContext('2d');
myCanvasContext.drawImage(image,0,0);
// get google chart gague to base64, yey!
var base64String = myCanvas.toDataURL();
Thanks to the author of this answer and this post