If you're looking for a Google Maps-style zoom, then I'm afraid you're out of luck; the GeoChart does not support that. If that is a requirement, you might be better off using the
Map visualization.
However, the GeoChart does support rendering at different resolutions (i.e. world level, continent level, country level, etc.), which you can use to make your own zoom functionality. Here's an example of this:
http://jsfiddle.net/gqzyysd5/
Note that it might be better to provide a 'Zoom out' button and/or breadcrumbs, but that would have been too much effort for a simple demo =)