property Chart.SelectDate(Date as Date) as Boolean
Selects or unselects a specific date in the chart.

Date as Date A DATE expression that indicates the 
Boolean A Boolean expression that specifies whether the Date is selected or not.
Use the SelectDate property to select dates programmatically. The SelectedDates property can be used to retrieve all selected dates, or to select a collection of dates. Use the UnselectDates method to unselect all dates in the chart. Use the SelectDate property to select or unselect a new date, or to find if a specified date is selected or it is not selected. The user can select dates by clicking the chart's header. Use the SelectLevel property to specify the area being highlighted when a date is selected. You can select multiple dates keeping the CTRL key and clicking a new date. The MarkSelectDateColor property specifies the color being used to highlight the selected dates. If the MarkSelectDateColor property is identical with the BackColor property of the Chart object, the selected dates are not shown.  The ChartEndChanging(exSelectDate) event notifies your application when the user selects a new date by clicking the header of the chart. Use the MarkTimeZone method to highlight different time-zones.

In the following screen shot the red lines marks the selected dates ( June 20 and June 28 ):