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: 

• UnitWidth, indicates the width of the base time-scale, in pixels. The MinUnitWidth property indicates the minimum value for the UnitWidth property when resizing/enlarging is performed. The MaxUnitWidth property indicates the maximum value for the UnitWidth property when resizing/enlarging is performed.
• UnitScale, indicates the time-scale unit for the chart. This property is changed ONLY if the AllowResizeChart property includes the exAllowChangeUnitScale flag.
• FirstVisibleDate, indicates the first visible date in the chart.

Here's some cases of the AllowResizeChart property values:

• exAllowResizeChartHeader, the user can resize the chart, by increasing or decreasing the chart's unit width by dragging the chart's header. 
• exAllowResizeChartHeader + exAllowChangeUnitScale, the user can zoom-in or zoom-out the chart, and the UnitScale property is changed. For instance, if the control's UnitWidth reaches the MinUnitWidth, the UnitScale property is changed to the next time-unit available. If the UnitWidth reaches the MaxUnitWidth, the UnitScale property is changed to the prev time-unit available.
• exAllowResizeChartHeader + exAllowResizeChartMiddle + exAllowChangeUnitScale, the user can zoom-in/zoom-out the chart using the control's header as well as clicking the middle mouse button of the control.

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.