  plenv(xmin, xmax, ymin, ymax, just, axis)
  *****************************************

  plenv

  Sets up plotter environment for simple graphs by calling pladv and
  setting up viewport and window to sensible default values. plenv leaves
  enough room around most graphs for axis labels and a title. When these
  defaults are not suitable, use the individual routines plvspa, plvpor, or
  plvasp for setting up the viewport, plwind for defining the window, and
  plbox for drawing the box.

  xmin (PLFLT, input): Value of x at left-hand edge of window.

  xmax (PLFLT, input): Value of x at right-hand edge of window.

  ymin (PLFLT, input): Value of y at bottom edge of window.

  ymax (PLFLT, input): Value of y at top edge of window.

  just (PLINT, input): If just=0, the x and y axes are scaled
  independently to use as much of the screen as possible, but if just=1, the
  scales of the x and y axes are made equal.

  axis (PLINT, input): Controls drawing of the box around the plot:

   o -2: No box or annotation.
   o -1: Draw box only.
   o 0: Draw box, labelled with coordinate values around edge.
   o 1: In addition to box and labels, draw the two axes x=0 and y=0.
   o 2: As for axis=1, but also draw a grid at the major tick interval.
   o 10: Logarithmic x axis, linear y axis.
   o 11: Logarithmic x axis, linear y axis and draw line y=0.
   o 20: Linear x axis, logarithmic y axis.
   o 21: Linear x axis, logarithmic y axis and draw line x=0.
   o 30: Logarithmic x and y axes.

