This event occurs immediately before a partial chart update request is sent to the server (see JsChartViewer.partialUpdate).

Use JsChartViewer.attachHandler with the event id "PreUpdate" to attach a handler to this event.

This event is commonly used to include custom information to be sent to the server (by storing them as custom attributes using JsChartViewer.setCustomAttr), such as the states of HTML controls in the web page.