This is the plot method for the class "envelope"
  of simulation envelopes. Objects of this class are
  created by the command envelope.
This plot method is currently identical to plot.fv.
Its default behaviour is to shade the region
  between the upper and lower envelopes in a light grey colour.
  To suppress the shading and plot the upper and lower envelopes
  as curves, set shade=NULL.
  To change the colour of the shading, use the argument shadecol
  which is passed to plot.fv.
  
See plot.fv for further information on how to
  control the plot.