docstr: get extreme values.
  
  x1, y1, x2, y2 in image coordinates (0-based)
  nx, ny : number of division in each axis
code: def __call__(self, transform_xy, x1, y1, x2, y2):
      """
      get extreme values.
  
      x1, y1, x2, y2 in image coordinates (0-based)
      nx, ny : number of division in each axis
      """
      # lon_min, lon_max, lat_min, lat_max = self._extremes
      return self._extremes
  
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  import six
  from six.moves import zip
  import numpy
  from matplotlib.transforms import Affine2D, IdentityTransform
  from .None import grid_helper_curvelinear
  from .axislines import AxisArtistHelper, GridHelperBase
  from .axis_artist import AxisArtist
  from .grid_finder import GridFinder
  from .grid_finder import ExtremeFinderSimple
  from .axislines import Axes
  from mpl_toolkits.axes_grid1.parasite_axes import host_axes_class_factory
  import matplotlib.axes
label_desc: Use image node_id 2 for calling the ExtremeFinderFixed obj's underlying member method code with example usage: obj.__call__(transform_xy, x1, y1, x2, y2) and returns: self