_A_d_d _a_n _A_x_i_s _t_o _a _P_l_o_t
	
	     axis(which, at, labels=TRUE, ...)
	
	_A_r_g_u_m_e_n_t_s:
	
	       which : an integer specifying which side of the plot
	               the axis is to be drawn on.
	
	          at : the points at which tick-marks are to be
	               drawn.
	
	      labels : this can either be a logical value specifying
	               whether (numerical) annotations are to be
	               made at the tickmarks, or a vector of charac-
	               ter strings to be placed at the tickpoints.
	
	         ... : graphical parameters may also be passed as
	               aruments to this function.
	
	_V_a_l_u_e:
	
	     This function is invoked for its side effect, which is
	     to add an axis to an already existing plot.  The axis
	     is placed as follows: 1=below, 2=left, 3=above and
	     4=right.
	
	_E_x_a_m_p_l_e_s:
	
	     plot(1:4, rnorm(4), axes=FALSE)
	     axis(1, 1:4, LETTERS[1:4])
	     axis(2)
	     box() #- to make it look "as usual"
	
	     ##-- This still fails in older versions of R :
	     plot(1:7, rnorm(7), type = 's', xaxt='n', col = 'red')
	     axis(1, 1:7, LETTERS[1:7], col.axis = 'blue')