Time range selection of Trend control in WinCC 6.0

Hi,

I would like to select the time range of a trend control from outside in WinCC 6.0.
(i.e, i don't want to use the time range selection tool button on the trend control itself).

I have a PDL named "Trends" and in that I have a trend control called "TrendCtrl" which displays archive trends.

I have used the following small sample script to do the same on the click of a button:

Dim objscreen, objitem
Set objscreen = HMIRuntime.Screens("Trends")
Set objitem=objscreen.ScreenItems("TrendCtrl")

objitem.index=0

objitem.TimeRange=False
objitem.BeginTime="01/03/2007 11:27:00 AM"
objitem.EndTime="01/03/2007 11:31:00 AM"

When I try this, the time axis gets updated properly but i don't get any graph display. If I switch on the ruler then I get the value in ruler as "###.###".
But if i select the same time range from the trend control's time selection button, then I get a proper display of the graph.

Is there any refresh method for trend control?

Thanks & Regards,
Manju