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.
    
      For greater control over the size of the plots, axis labelling and
      tick intervals, more complex graphs should make use of the functions
      plvpor, plvasp, plvpas, plwind, plbox, and routines for
      manipulating axis labelling plgxax through plszax.