This is a nice question. It is possible, a little tricky but possible, for example we want to register animationfinish event, we'll do this:

BarChart::create([
    "name"=>"myBarChart",
    "onReady"=>"function(){
        google.visualization.events.addListener(myBarChart.chart, 'animationfinish', function(){
            //Do something here
        });
    }"
])

Hope that helps.

Since google chart object is loaded by a loader js users might want to check if it's loaded yet before adding custom events. Pls try this onReady function:

        ...
        'onReady' => "function() {
            function isGoogleChartLoaded() {
                if(typeof google === 'undefined') {
                    console.log('google chart not loaded yet');
                    window.setTimeout(isGoogleChartLoaded, 100); /* check if google chart is loaded every 100 milliseconds*/
                } else {
                    console.log('google chart loaded');
                    addCustomGoogleChartEvents();
                }
            }
            isGoogleChartLoaded();
        }"