Registers a callback that's fired when the chart is clicked.
Returns an ID which can be passed to unlisten() to unregister the callback.
|ui.Chart||The ui.Chart instance.|
|Function||The callback to fire when the chart is clicked. The callback is passed three arguments: the x-value, the y-value, and the series name. Time values are represented in UTC epoch milliseconds, like "system:time_start" values on assets. If the user clicks on a legend entry to select an entire series, the x- and y-values are null. If the user clicks an already-selected point, all arguments are null, indicating the selection was cleared.|