Thanx a lot spitfire,
its great help
The vertical marker is now slaved to the x axis. And as suggested by you i am able to add a circle , the position of which can be varied using x and y values.
However there is some problem regarding the size of the circle. The maximum size of the circle can be equal to the width of the Vertical line and to make circle noticeable i have to increase the size of the vertical bar. As a result the vertical bar becomes wider and hides the significant portion of the graph. when i try to just increase the size of the circle without increasing the size of vertical bar, the circle is clipped and only a portion of it is visible on the bar.
I need to display a thin vertical bar with a noticeable circle as shown in your demo pic.
Kindly suggest how to achieve this.

Regards
Raghav