Registers a callback that's fired when the chart is clicked.
Returns an ID which can be passed to unlisten() to unregister the callback.
The ui.Chart instance.
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.
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