class_docstr: Class used to plot points on the manifold SPD(2).
  
  Elements S of the manifold of 2D Symmetric Positive Definite matrices
  can be conveniently represented by ellipses.
  
  We write :math:`S = O D O^T` with :math:`O` an orthogonal matrix (rotation)
  and :math:`D` a diagonal matrix. The positive eigenvalues, i.e. the elements of
  :math:`D`, are the inverse of the length of the major and minor axes of
  the ellipse. The rotation matrix :math:`O` determines the orientation of the
  2D ellipse in the 2D plane.
  
  Parameters
  ----------
  k_sampling_points : int
      Number of points to sample on the discretized ellipses.
docstr: Set the axis for the Figure.
  
  Parameters
  ----------
  ax : Axis
      Axis of the figure.
  
  Returns
  -------
  ax : Axis
      Axis of the figure.
docstr_tok: ["Set","the","axis","for","the","Figure",".","Parameters","--","--","--","--","--","ax",":","Axis","Axis","of","the","figure",".","Returns","--","--","--","-","ax",":","Axis","Axis","of","the","figure","."]
code: def set_ax(ax=None):
      """Set the axis for the Figure.
  
      Parameters
      ----------
      ax : Axis
          Axis of the figure.
  
      Returns
      -------
      ax : Axis
          Axis of the figure.
      """
      if ax is None:
          ax = plt.subplot()
      plt.setp(ax, xlabel="X", ylabel="Y")
      return ax
  
imports: import matplotlib.pyplot
  from mpl_toolkits.mplot3d import Axes3D
  import geomstats.backend
label_desc: Use image node_id 2 for calling the Ellipses obj's underlying member method code with example usage: obj.set_ax(ax) and returns: ax