docstr: return Nx2 array of x,y coordinate of the boundary
code: def get_boundary(self):
      """
      return Nx2 array of x,y coordinate of the boundary
      """
      x0, x1, y0, y1 = self._extremes
      tr = self._aux_trans
      xx = np.linspace(x0, x1, 100)
      yy0, yy1 = np.empty_like(xx), np.empty_like(xx)
      yy0.fill(y0)
      yy1.fill(y1)
  
      yy = np.linspace(y0, y1, 100)
      xx0, xx1 = np.empty_like(yy), np.empty_like(yy)
      xx0.fill(x0)
      xx1.fill(x1)
  
      xxx = np.concatenate([xx[:-1], xx1[:-1], xx[-1:0:-1], xx0])
      yyy = np.concatenate([yy0[:-1], yy[:-1], yy1[:-1], yy[::-1]])
      t = tr.transform(np.array([xxx, yyy]).transpose())
  
      return t
  
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 6 for calling the GridHelperCurveLinear obj's underlying member method code with example usage: obj.get_boundary() and returns: t