You cannot access the btnDD because it is defined in a DataTemplate and the framework does not allow you to access this element because you might use the template in many places and you will have many btnDD buttons.
By default the track ball info control has its IsHitTestVisible property set to false. This is to prevent the track ball from hiding when the mouse is over it. If the hit test property is set to true, the track info control will swallow the mouse events and this will trigger the mouse leave event of the plot area, which in turn will hide the track ball, so you will not be able to click the button.
Perhaps you can remove the track ball behavior and use annotations instead. Check out our track-ball-like-annotations sdk example
. You may want to use the sdk browser
for easier browsing. The example demonstrates how you can use annotations to mimic a track-ball-like feature. You can then update this example and place a button which you can click. I hope this helps.
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