Hi Jeff,
Greetings from Syncfusion.
We have checked your query at our end, and we would like to let you know that onZooming callback event only hits when the panning or pinching is performed in the chart widget. When performing mouse wheel zooming in the chart, the callback events onZoomStart, onZoomEnd and onZoomReset can be used for tracking the zooming related values. Please check and get back to us if you require further assistance.
Regards,
Sriram Kiran