wn_plot_fill(_c)

	status.i4.v = wn_plot_fill(on_off.i4.r, base_level.r4.r)

	status.i4.v = wn_plot_fill_c(on_off.i4.v, base_level.r4.v)

	Sets the state of filling on plots for the active window. 
	This will control if the plot is filled-in between the plotted curve
	and the "base_level".  A previous call to wn_active(_c) is required.
	The color is determined by the latest call to wn_color(_c).

	on_off		fill flag
			(TRUE -> fill under curve (turns off connect points),
			 FALSE -> do not fill under curve,
			 CLIB_DEFAULT_ARG -> simply return the present
					     fill state)
	base_level	real-world y-value that sets the border of the fill

	This function returns ACNET status values as follows:

	CBS_INVARG		invalid state specified
	CBS_NOSETUP		an active window has not been established
	otherwise		previous fill state

	This function requires the following include files:

	cnsparam_h, cbslib_h, acnet_errors_h

	Related functions:

	wn_connect_points(_c), wn_point(_c), wn_symbol(_c), wn_color(_c),
	wn_user_defined_color_c, wn_read_user_defined_color,
	intro_graphics_windows

	C/C++ usage:

	int	status;
	int	on_off = TRUE;
	float	base_level = 0.0;

	status = wn_plot_fill_c(on_off,base_level);