docs: def plot_powerlaw(exp, startx, starty, width=None, **kwargs):
    r'''
  Plot a power-law.
  
  :arguments:
  
    **exp** (``float``)
      The power-law exponent.
  
    **startx, starty** (``float``)
      Start coordinates.
  
  :options:
  
    **width, height, endx, endy** (``float``)
      Definition of the end coordinate (only on of these options is needed).
  
    **units** ([``'relative'``] | ``'absolute'``)
      The type of units in which the coordinates are specified. Relative coordinates correspond to a
      fraction of the relevant axis. If you use relative coordinates, be sure to set the limits and
      scale before calling this function!
  
    **axis** ([``plt.gca()``] | ...)
      Specify the axis to which to apply the limits.
  
    ...
      Any ``plt.plot(...)`` option.
  
  :returns:
  
    The handle of the ``plt.plot(...)`` command.
    '''
  
    # get options/defaults
    endx   = kwargs.pop('endx'  , None      )
    endy   = kwargs.pop('endy'  , None      )
    height = kwargs.pop('height', None      )
    units  = kwargs.pop('units' , 'relative')
    axis   = kwargs.pop('axis'  , plt.gca() )
  
    # check
    if axis.get_xscale() != 'log' or axis.get_yscale() != 'log':
      raise IOError('This function only works on a log-log scale, where the power-law is a straight line')
  
    # apply width/height
    if width is not None:
  
      endx = startx + width
      endy = None
  
    elif height is not None:
  
      if   exp >  0: endy = starty + height
      elif exp == 0: endy = starty
      else         : endy = starty - height
  
      endx = None
  
    # transform
    if units.lower() == 'relative':
      [startx, endx] = rel2abs_x([startx, endx], axis)
      [starty, endy] = rel2abs_y([starty, endy], axis)
  
    # determine multiplication constant
    const = starty / ( startx**exp )
  
    # get end x/y-coordinate
    if endx is not None: endy = const * endx**exp
    else               : endx = ( endy / const )**( 1/exp )
  
    # plot
    return axis.plot([startx, endx], [starty, endy], **kwargs)