docs: def plot(x, y, units='absolute', axis=None, **kwargs):
    r'''
  Plot.
  
  :arguments:
  
    **x, y** (``list``)
      Coordinates.
  
  :options:
  
    **units** ([``'absolute'``] | ``'relative'``)
      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!
  
    ...
      Any ``plt.plot(...)`` option.
  
  :returns:
  
    The handle of the ``plt.plot(...)`` command.
    '''
  
    # get current axis
    if axis is None:
      axis = plt.gca()
  
    # transform
    if units.lower() == 'relative':
      x = rel2abs_x(x, axis)
      y = rel2abs_y(y, axis)
  
    # plot
    return axis.plot(x, y, **kwargs)