This behavior is caused by the fact that chart's behaviors does not handle mouse wheel events. In order to achieve your expected result please attache to charts MouseWheel event like shown below:
void chart_MouseWheel(object sender, MouseWheelEventArgs e)
var pos = e.GetPosition(this.chart);
ChartPanAndZoomBehavior behav = (ChartPanAndZoomBehavior)this.chart.Behaviors.FirstOrDefault(b => b is ChartPanAndZoomBehavior);
bool isZoom = behav != null;
e.Handled = true;
This way when the mouse is over the chart and the wheel is being moved forward/backward, the zooming will happen. When the mouse is over the ItemsControl's scrollbar, then only the ItemsControl will scroll.
Hope it helps.
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