code: def plot_envelope(M, C, mesh):
      
  
      try:
          from pylab import fill, plot, clf, axis
          x = concatenate((mesh, mesh[::-1]))
          mean, var = point_eval(M, C, mesh)
          sig = sqrt(var)
          mean = M(mesh)
          y = concatenate((mean - sig, (mean + sig)[::-1]))
          # clf()
          fill(x, y, facecolor='.8', edgecolor='1.')
          plot(mesh, mean, 'k-.')
      except ImportError:
          print_("Matplotlib is not installed; plotting is disabled.")
docs: plot_envelope(M,C,mesh)
  
  
      plots the pointwise mean +/- sd envelope defined by M and C
      along their base mesh.
  
  
      :Arguments:
  
          -   `M`: A Gaussian process mean.
  
          -   `C`: A Gaussian process covariance
  
          -   `mesh`: The mesh on which to evaluate the mean and cov.