It looks like it depends on when you call GetGraphCursor. If you call it on EVENT_LEFTCLICK, you get the clicked-on coordinate. If you call it on EVENT_COMMIT, you get the snapped-to coordinate.
I just tried the graphcursors.prj sample project that shipped with CVI using CVI 6.0 and 7.0 (evaluation). In both cases, GetGraphCursor returned the X and Y coordinates to which the cursor snapped, not where I clicked.
In the sample, the Marker Position is for cursor 2, the short cross marker (set to snap to grid). I ran the project, clicked on Plot Data, moved cursor 3 (one of the long-cross markers) to a marked point on the Y axis, then moved cursor 2 along the horizontal line of cursor 3 and clicked at various points. That way I could easily tell that the Y coordina
te was the snapped-to coordinate, not the clicked coordinate.
If you change the callback for the graph to use EVENT_LEFTCLICK, you get the clicked-on coordinate.