While searching for an example of EnableExtendedMouseEvents I got in touch with the attached sample program that does exactly what you are searching for: it updates a graph while a button is continously pressed. You won't be able to execute it since it's for CVI2009 (you can look if it exists a version for your release on your disk: I would search in the help for the command), but you can look at MoveCB () function to understand how it works. Basically it intercepts EVENT_LEFT_CLICK to mark the start of the mouse press and EVENT_LEFT_MOUSE_UP to mark press end.
You can also look at the example shown on this page: it's rather obscure since you can only have program output in the debug window but it's another piece of code to study. To execute it you will probably need to rebuild the user interface in CVI5.5: it's basically a simple panel (name: PANEL) with a canvas inside (name: CANVAS; callback: CanvasEvents) and a quit button (name: QUITBUTTON; callback: QuitCallback).
** EDIT ** I already pointed you to the second example some posts ago: I leave it since I am adding some infos tha could help you to to run it in your system.
