Hi Raymond,
I am afraid this feature has not yet been implemented in chartControl. I will create a feature request on this and will update you as soon as the feature is implemented.
I have done a workaround for drop lines and attached sample for you. Yes It is possible to display the (x,y) values for CustomPoint.
Let me know if this fulfills your requirement.
Thanks for your interest in Syncfusion product.
Regards,
F.Rashidha.
ChartGetValueSample.zip