pzmap(sys, grid=True) fails #457

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-7-4e6a52bd77da> in <module>
      1 import control
      2 g = control.tf(1, [1,1])
----> 3 control.pzmap(g, grid=True)

~/src/python-control/control/pzmap.py in pzmap(sys, plot, grid, title, **kwargs)
    104                 ax, fig = zgrid()
    105             else:
--> 106                 ax, fig = sgrid()
    107         else:
    108             ax, fig = nogrid()

~/src/python-control/control/grid.py in sgrid()
     80     ax.axis[:].invert_ticklabel_direction()
     81 
---> 82     ax.axis["wnxneg"] = axis = ax.new_floating_axis(0, 180)
     83     axis.set_ticklabel_direction("-")
     84     axis.label.set_visible(False)

~/.miniconda3/envs/control-dev/lib/python3.8/site-packages/mpl_toolkits/axisartist/axislines.py in new_floating_axis(self, nth_coord, value, axis_direction)
    571     def new_floating_axis(self, nth_coord, value, axis_direction="bottom"):
    572         gh = self.get_grid_helper()
--> 573         axis = gh.new_floating_axis(nth_coord, value,
    574                                     axis_direction=axis_direction,
    575                                     axes=self)

~/.miniconda3/envs/control-dev/lib/python3.8/site-packages/mpl_toolkits/axisartist/grid_helper_curvelinear.py in new_floating_axis(self, nth_coord, value, axes, axis_direction)
    348             self, nth_coord, value, axis_direction)
    349 
--> 350         axisline = AxisArtist(axes, _helper)
    351 
    352         # _helper = FloatingAxisArtistHelper(self, nth_coord,

~/.miniconda3/envs/control-dev/lib/python3.8/site-packages/mpl_toolkits/axisartist/axis_artist.py in __init__(self, axes, helper, offset, axis_direction, **kwargs)
    744         self._axis_direction = axis_direction
    745 
--> 746         self._init_line()
    747         self._init_ticks(**kwargs)
    748         self._init_offsetText(axis_direction)

~/.miniconda3/envs/control-dev/lib/python3.8/site-packages/mpl_toolkits/axisartist/axis_artist.py in _init_line(self)
    885         if axisline_style is None:
    886             self.line = PathPatch(
--> 887                 self._axis_artist_helper.get_line(self.axes),
    888                 color=rcParams['axes.edgecolor'],
    889                 fill=False,

~/.miniconda3/envs/control-dev/lib/python3.8/site-packages/mpl_toolkits/axisartist/grid_helper_curvelinear.py in get_line(self, axes)
    268 
    269     def get_line(self, axes):
--> 270         self.update_lim(axes)
    271         x, y = self.grid_info["line_xy"]
    272 

~/.miniconda3/envs/control-dev/lib/python3.8/site-packages/mpl_toolkits/axisartist/grid_helper_curvelinear.py in update_lim(self, axes)
     99 
    100     def update_lim(self, axes):
--> 101         self.grid_helper.update_lim(axes)
    102 
    103         x1, x2 = axes.get_xlim()

~/.miniconda3/envs/control-dev/lib/python3.8/site-packages/mpl_toolkits/axisartist/axislines.py in update_lim(self, axes)
    325 
    326         if self._force_update or self._old_limits != (x1, x2, y1, y2):
--> 327             self._update(x1, x2, y1, y2)
    328             self._force_update = False
    329             self._old_limits = (x1, x2, y1, y2)

~/.miniconda3/envs/control-dev/lib/python3.8/site-packages/mpl_toolkits/axisartist/grid_helper_curvelinear.py in _update(self, x1, x2, y1, y2)
    317         if self.valid() and self._old_values == (x1, x2, y1, y2):
    318             return
--> 319         self._update_grid(x1, y1, x2, y2)
    320         self._old_values = (x1, x2, y1, y2)
    321         self._force_update = False

~/.miniconda3/envs/control-dev/lib/python3.8/site-packages/mpl_toolkits/axisartist/grid_helper_curvelinear.py in _update_grid(self, x1, y1, x2, y2)
    365 
    366     def _update_grid(self, x1, y1, x2, y2):
--> 367         self.grid_info = self.grid_finder.get_grid_info(x1, y1, x2, y2)
    368 
    369     def get_gridlines(self, which="major", axis="both"):

~/.miniconda3/envs/control-dev/lib/python3.8/site-packages/mpl_toolkits/axisartist/grid_finder.py in get_grid_info(self, x1, y1, x2, y2)
    102         """
    103 
--> 104         extremes = self.extreme_finder(self.inv_transform_xy, x1, y1, x2, y2)
    105 
    106         # min & max rage of lat (or lon) for each grid line will be drawn.

~/src/python-control/control/grid.py in __call__(self, transform_xy, x1, y1, x2, y2)
     42 
     43         lon_min, lon_max, lat_min, lat_max = \
---> 44             self._adjust_extremes(lon_min, lon_max, lat_min, lat_max)
     45 
     46         return lon_min, lon_max, lat_min, lat_max

AttributeError: 'ModifiedExtremeFinderCycle' object has no attribute '_adjust_extremes'