Do you want a link opened when someone clicks a data point, or do you want a link added to the popup?
If you want a link opened on click, after you call draw() on your chart object, add an event listener for the "select" event and use js to direct your navigation from there:
// assuming you have a chart object called "chart"
google.events.addListener(chart, 'select', function () {
// whatever you want, ex:
});
If you want the popups to have links in them, you need to set the formatted values of your data. Either call the dataTable method #setFormattedValue for each point, or set your values and formatted values in object notation when you build your dataTable:
// assuming you have a dataTable object called "data"