The easiest way I found to manage using the same event listener function for multiple charts is to put the charts in an array and loop over the array, assigning an event listener for each chart:
// charts is an array of charts
for (var i = 0; i < charts.length; i++) {
google.visualization.events.addListener(charts[i], 'ready', (function (x) {
return function () {
// use charts[x] to access the specific chart
}
})(i);
}
Note that you need to use a closure around the function to map each instance of "i" to the handler function for that chart.