The function plenv is used to define the scales and axes for simple
      graphs.  plenv starts a new picture on the next subpage (or a new
      page if necessary), and defines the ranges of the variables required.
      The routine will also draw a box, axes, and numeric labels if
      requested.  The syntax for plenv is:
    
      
| 	  plenv
	(xmin, xmax, ymin, ymax, just, axis); | 
| xmin, xmax
	  (PLFLT, input)	    The left and right limits for the horizontal axis.
	  ymin, ymax
	  (PLFLT, input)	    The bottom and top limits for the vertical axis.
	  just
	  (PLINT, input)	    This should be zero or one.  If just is
	    one, the scales of the x-axis and y-axis will be the same (in
	    units per millimeter); otherwise the axes are scaled
	    independently.  This parameter is useful for ensuring that
	    objects such as circles have the correct aspect ratio in the
	    final plot.
	  axis
	  (PLINT, input)	    axis controls whether a box, tick marks,
	    labels, axes, and/or a grid are drawn.
	   		axis = -2: No box or annotation.
	      		axis = -1: Draw box only.
	      		axis = 0: Draw box, labeled with
		coordinate values around edge.
	      		axis = 1: In addition to box and labels,
		draw the two axes X = 0 and Y = 0.
	      		axis = 2: Same as
		axis = 1, but also draw a grid at the
		major tick interval.
	      		axis = 10: Logarithmic X axis, linear Y
		axis.
	      		axis = 11: Logarithmic X axis, linear Y
		axis and draw line Y = 0.
	      		axis = 20: Linear X axis,
		logarithmic Y axis.
	      		axis = 21: Linear X axis, logarithmic Y
		axis and draw line X = 0.
	      		axis = 30: Logarithmic X and Y axes.
	      
 | 
      Note: Logarithmic axes only affect the appearance of the axes and
      their labels, so it is up to the user to compute the logarithms prior
      to passing them to plenv and any of the other routines.  Thus, if a
      graph has a 3-cycle logarithmic axis from 1 to 1000, we need to set
      xmin =
      log10(1) = 0.0, and
      xmax =
      log10(1000) = 3.0.
    
      For greater control over the size of the plots, axis labeling and
      tick intervals, more complex graphs should make use of the functions
      plvpor, plvasp, plvpas, plwind, plbox, and routines for
      manipulating axis labeling plgxax through plszax.