docstr: create a twin of Axes for generating a plot with a shared
  y-axis but independent x axis.  The x-axis of self will have
  ticks on bottom and the returned axes will have ticks on the
  top
code: def twiny(self, axes_class=None):
      """
      create a twin of Axes for generating a plot with a shared
      y-axis but independent x axis.  The x-axis of self will have
      ticks on bottom and the returned axes will have ticks on the
      top
      """
  
      if axes_class is None:
          axes_class = self._get_base_axes()
  
      parasite_axes_class = parasite_axes_class_factory(axes_class)
  
      ax2 = parasite_axes_class(self, sharey=self, frameon=False)
      self.parasites.append(ax2)
  
      self.axis["top"].set_visible(False)
  
      ax2.axis["top"].set_visible(True)
      ax2.axis["left", "right", "bottom"].set_visible(False)
  
      def _remove_method(h):
          self.parasites.remove(h)
          self.axis["top"].set_visible(True)
          self.axis["top"].toggle(ticklabels=False, label=False)
  
      ax2._remove_method = _remove_method
  
      return ax2
  
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  import six
  from matplotlib import artist, collections, transforms, rcParams
  from matplotlib.axes import subplot_class_factory
  from matplotlib.transforms import Bbox
  from .mpl_axes import Axes
  import numpy
label_desc: Use image node_id 7 for calling the HostAxesBase obj's underlying member method code with example usage: obj.twiny(axes_class) and returns: ax2