property Chart.AllowResizeChart as ResizeChartEnum
Specifies whether the user can enlarge (zoom-in,zoom-out) or resize the chart using the control's header, middle mouse button.

ResizeChartEnum A ResizeChartEnum expression that indicates the way user can resize or enlarge the control's chart.
By default, the AllowResizeChart property is exDisableResizeChart, so the user is not able to perform any enlargement or zooming using the control's header or middle mouse button. The AllowResizeChart property allows the user to resize or enlarge the chart at runtime. The ChartStartChanging/ChartEndChaning events are fired to notify your application that the user starts or ends resizing/enlarging the chart. Here's a short presentation of how the resizing/enlarging could work. The user can resize the chart by drag and drop the left or right resize-margins of the overview-selection, while the Background(exOverviewSelResize) property is not zero.

When the user resizes or enlarges the chart at runtime, the following properties may be changed: 

Here's some cases of the AllowResizeChart property values:

In conclusion, if the AllowResizeChart property includes no exAllowChangeUnitScale flag, the UnitScale property of the chart is not changed while resizing is performed, so actually only the UnitWidth and FirstVisibleDate may be changed. If the AllowResizeChart property includes exAllowChangeUnitScale flag, all of the mentioned properties may be changed. The Label property of the Chart indicates the available time-scale units when zoom-in zoom-out is performed. For instance, if the Label(exHour) property is empty, the exHour time-scale unit is not available, else If the Label(exHour) is not empty, the exHour time scale unit is available. In other words, when zoom-in/zoom-out is performed the control's chart can be zoomed to exHour only if the Chart.Label(exHour) property is not empty.