property Chart.OverviewSelTransparent(Items as Boolean) as Long
Specifies the percent of the transparency to display the selection in the overview parts of the control.

Items as Boolean A Boolean expression that specifies whether the transparency for selection is changed in the items or scale-zooming part. 
Long A long expression that indicates the transparency to show the selection in the overview part of the control.  The value should be from 0 to 100, as 0 opaque, 50 - semi-transparent , 100 fully transparent ( not shown )
By default, the OverviewSelTransparent property is 0, which means that the selection is opaque. The OverviewVisible property specifies whether the chart's overview map is visible or hidden. Use the OverviewSelBackColor property to change the visual appearance of the selection in the overview area. The OverviewBackColor property specifies the background color for the overview area. The overview-map part can display the scale-zooming scale if the AllowOverviewZoom property is not exDisableZoom.

The following screen shots shows the overview-map part of the control with different values for the OverviewSelTransparent property:

OverviewSelTransparent(True) = 0, OverviewSelTransparent(False) = 0

OverviewSelTransparent(True) = 70, OverviewSelTransparent(False) = 0

OverviewSelTransparent(True) = 0, OverviewSelTransparent(False) = 70