docs: def rel2abs_x(x, axis=None):
    r'''
  Transform relative x-coordinates to absolute x-coordinates. Relative coordinates correspond to a
  fraction of the relevant axis. Be sure to set the limits and scale before calling this function!
  
  :arguments:
  
    **x** (``float``, ``list``)
      Relative coordinates.
  
  :options:
  
    **axis** ([``plt.gca()``] | ...)
      Specify the axis to which to apply the limits.
  
  :returns:
  
    **x** (``float``, ``list``)
      Absolute coordinates.
    '''
  
    # get current axis
    if axis is None:
      axis = plt.gca()
  
    # get current limits
    xmin, xmax = axis.get_xlim()
  
    # transform
    # - log scale
    if axis.get_xscale() == 'log':
      try   : return [10.**(np.log10(xmin)+i*(np.log10(xmax)-np.log10(xmin))) if i is not None else i for i in x]
      except: return  10.**(np.log10(xmin)+x*(np.log10(xmax)-np.log10(xmin)))
    # - normal scale
    else:
      try   : return [xmin+i*(xmax-xmin) if i is not None else i for i in x]
      except: return  xmin+x*(xmax-xmin)