_C_o_n_d_i_t_i_o_n_i_n_g _P_l_o_t_s
	
	     coplot(formula, data, given.values, rows, columns,
	             show.given=TRUE, col=par("fg"), pch=par("pch"), ...)
	     co.intervals(x, number=6, overlap=0.5)
	
	_A_r_g_u_m_e_n_t_s:
	
	     formula : a formula describing the form of conditioning
	               plot.  A formula of the form y~x|a indicates
	               that plots of y versus x should be produced
	               conditional on the variable.  A formula of
	               the form y~x|a*b indicates that plots of y
	               versus x should be produced conditional on
	               the two variables a and b.  x and lang(y)
	               must be numeric, but a and b may be either
	               numeric or factors.
	
	        data : a data frame containing values for any vari-
	               ables in the formula.  By default the
	               environment where coplot was called from is
	               used.
	
	given.values : a value or list of two values which determine
	               how the conditioning on a and b is to take
	               place. Usually this is a matrix with two
	               columns which each row of which gives and
	               interval, to be conditioned on, but is can
	               also be a single vector of numbers or a set
	               of factor levels (if the variable being con-
	               ditioned on is a factor).
	
	       panel : a function which gives the action to be car-
	               ried out in each panel of the display.  The
	               default is points.
	
	        rows : the panels of the plot are laid out in a rows
	               by columns array.  rows gives the number of
	               rows in the array.
	
	     columns : the number of columns in the panel layout
	               array.
	
	  show.given : should conditioning plots be shown for the
	               conditioning variables (default true)
	
	         col : a vector of colors to be ised to plot the
	               points.  If too short, the values are recy-
	               cled.
	
	         pch : a vector of plotting symbols or characters.
	               If too short, the values are recycyled.
	
	      number : the number conditioning intervals.
	
	     overlap : the fraction of overlap of the conditioning
	               variables.
	
	_D_e_s_c_r_i_p_t_i_o_n:
	
	     This function produces two variants of the conditioning
	     plots discussed in the reference.
	
	_R_e_f_e_r_e_n_c_e_s:
	
	     Cleveland, W. S. (1993).  Visualizing Data.  New Jer-
	     sey: Summit Press.
	
	_S_e_e _A_l_s_o:
	
	     pairs, panel.smooth, points.
	
	_E_x_a_m_p_l_e_s:
	
	     # Tonga Trench Earthquakes
	     data(quakes)
	     coplot(long ~ lat |depth, data=quakes)
	     coplot(long ~ lat | depth * mag, data=quakes)