GVH Custom MVVM and MouseEvents

I have a custom MVVM module built for GVH. I have hooked up the the mouse down event with intent of capturing a feature on the click.  With just a barebones handler and debugging in Chrome the event fires and the MouseEvent argument comes through with a graphic property.  Problem is, if I try to access the graphic property in my typescript code, that property is not valid as the MouseEvent argument is a more generic type and not one specific to GVH.