Question on exporting png file of google chart

[Keny]

Hello all, 

I am working on a webpage with a pie chart and bar chart.I want to create two hyperlinks to generate image files of the charts.

But I have troubles on it.

I took reference on https://developers.google.com/chart/interactive/docs/printing

But it is not quite complete.

My case is I still don't get the <div id="png"> worked. Any examples and ideas?