new_contents: from probfit.pdf import exponential
  from probfit.plotting import draw_pdf
  from collections import OrderedDict
  import matplotlib.pyplot as plt
  
  bound = (0, 5)
  
  arg = OrderedDict(tau=0.5)
  draw_pdf(exponential, arg=arg, bound=bound, label=str(arg), density=False,
           bins=100)
  
  arg = OrderedDict(tau=1.0)
  draw_pdf(exponential, arg=arg, bound=bound, label=str(arg), density=False,
           bins=100)
  
  arg = OrderedDict(tau=1.5)
  draw_pdf(exponential, arg=arg, bound=bound, label=str(arg), density=False,
           bins=100)
  
  plt.grid(True)
  plt.legend().get_frame().set_alpha(0.5)
  
subject: add doc/pyplots/pdf/exponential.py