status.i4.v = bpm_interrogate_plot(update_func.f.v [,screen.i4.v])

	This routine enables interrogating existing BPM plots created
	by calls to bpm_plot_data.  When the graphics cursor is in a
	BPM plot window, information about the data point closest to the
	cursor is passed to the update function specified in "update_func".

	update_func	function to be called each time a BPM plot data
			point is passed over
			(called as follows:
			 update_func(update_info.s.r, event_type.i4.v)
			 ("update_info" is a structure of type
	[screen]	graphics screen to interrogate
			(BPM_PLT_SCR_G1 -> GxP1,
			 BPM_PLT_SCR_G2 -> GxP2)
			(default (CLIB_DEFAULT_ARG) is last screen plotted on)

	This function returns ACNET status values as follows:

	OK			success
	CBS_INVARG		invalid screen requested
	CBS_NOINIT		no BPM plots are present
	CBS_DUPREQ		updating is already enabled

	This function requires the following include files:

	ul_cbsaux/bpmuti.h, acnet_errors_h

	Related functions:

	bpm_plot_data, bpm_get_plot_window_info_c, bpm_calculate_rms_dpoverp_c

	C/C++ usage:

	int	status;
	int	screen = BPM_PLT_SCR_G1;
	void update_func(BPM_PLOT_UPDATE_DATA *update_info, int event_type);
	status = bpm_interrogate_plot(update_func,screen);