Start to fix Attractors example

[aaronayres35]

fixes starts process of fixing #404

Previously changing initial_point at all one got the traceback:

Exception occurred in traits notification handler for event object: TraitChangeEvent(object=<attractors.model.henon.Henon object at 0x7fcc0ae66e60>, name='initial_point', old=array([0.1, 0.1]), new=array(['0.13', '0.1'], dtype='<U4'))
Traceback (most recent call last):
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/_trait_event_notifier.py", line 122, in __call__
    self.dispatcher(handler, event)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/traits/observation/observe.py", line 26, in dispatch_same
    handler(event)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/traits/has_traits.py", line 328, in handler
    instance.trait_property_changed(property_name, old)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/traits/has_traits.py", line 927, in decorator
    self.__dict__[name] = result = function(self)
  File "envisage/examples/demo/plugins/tasks/attractors/model/henon.py", line 68, in _get_points
    point = array([y + 1 - self.a * x ** 2, self.b * x])
TypeError: must be str, not int

Inserting a print statement (print(type(point), point.dtype, point)) I saw that :

<class 'numpy.ndarray'> float64 [0.1 0.1]
<class 'numpy.ndarray'> <U4 ['0.13' '0.1']

so by changing a value on initial_point we ended up changing the dtype leading to problems. We always want floats.

Note even after this fix there are still other problems with the example

[aaronayres35]

Now, I can change initial_point without an error message. however, if I set the y value of inital_point to 14 and then drag the plot I get:

$ python envisage/examples/demo/plugins/tasks/run_attractor.py 
2021-05-12 10:32:04.824 Python[7752:139557] ApplePersistenceIgnoreState: Existing state will not be touched. New state will be written to (null)
envisage/examples/demo/plugins/tasks/attractors/model/henon.py:68: RuntimeWarning: overflow encountered in double_scalars
  point = array([y + 1 - self.a * x ** 2, self.b * x])
/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/chaco/data_range_1d.py:285: FutureWarning: elementwise comparison failed; returning scalar instead, but in the future will perform elementwise comparison
  if self._high_setting != val:
/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/chaco/data_range_1d.py:292: FutureWarning: elementwise comparison failed; returning scalar instead, but in the future will perform elementwise comparison
  if val == 'auto':
/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/chaco/data_range_1d.py:298: FutureWarning: elementwise comparison failed; returning scalar instead, but in the future will perform elementwise comparison
  elif val == 'track':
Traceback (most recent call last):
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/enable/qt4/base_window.py", line 243, in paintEvent
    self.handler.paintEvent(event)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/enable/qt4/base_window.py", line 78, in paintEvent
    self._enable_window._paint(event)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/enable/abstract_window.py", line 542, in _paint
    self.component.draw(gc, view_bounds=(0, 0, size[0], size[1]))
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/enable/component.py", line 418, in draw
    self._draw(gc, view_bounds, mode)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/enable/component.py", line 789, in _draw
    self._dispatch_draw(layer, bb, view_bounds, mode)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/enable/container.py", line 280, in _dispatch_draw
    component._dispatch_draw(layer, gc, new_bounds, mode)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/enable/component.py", line 817, in _dispatch_draw
    handler(gc, view_bounds, mode)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/enable/component.py", line 910, in _draw_underlay
    underlay.overlay(self, gc, view_bounds, mode)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/chaco/axis.py", line 225, in overlay
    self._draw_component(gc, view_bounds, mode, component)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/chaco/axis.py", line 250, in _draw_component
    self._compute_tick_positions(gc, component)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/chaco/axis.py", line 496, in _compute_tick_positions
    scale=scale), float64)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/chaco/ticks.py", line 84, in get_ticks
    interval, use_endpoints=False), float64)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/chaco/ticks.py", line 216, in auto_ticks
    ticks = arange( start, end + (tick_interval / 2.0), tick_interval )
ValueError: arange: cannot compute length
Abort trap: 6

perhaps this should be a separate issue/ perhaps a chaco issue?

I tried running with chaco master as I remember a similar looking issue that had been fixed recently, only to find the following error:

$ python envisage/examples/demo/plugins/tasks/run_attractor.py 
2021-05-12 10:34:29.178 Python[7908:142952] ApplePersistenceIgnoreState: Existing state will not be touched. New state will be written to (null)
Traceback (most recent call last):
  File "envisage/examples/demo/plugins/tasks/run_attractor.py", line 26, in <module>
    main(sys.argv)
  File "envisage/examples/demo/plugins/tasks/run_attractor.py", line 16, in main
    app.run()
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/envisage/ui/tasks/tasks_application.py", line 178, in run
    self._create_windows()
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/envisage/ui/tasks/tasks_application.py", line 339, in _create_windows
    if not self._get_task_factory(task_id):
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/envisage/ui/tasks/tasks_application.py", line 362, in _get_task_factory
    for factory in self.task_factories:
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/envisage/extension_point.py", line 169, in get
    extensions = extension_registry.get_extensions(self.id)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/envisage/extension_registry.py", line 114, in get_extensions
    return self._get_extensions(extension_point_id)[:]
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/envisage/provider_extension_registry.py", line 103, in _get_extensions
    extensions = self._initialize_extensions(extension_point_id)
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/envisage/provider_extension_registry.py", line 289, in _initialize_extensions
    extensions.append(provider.get_extensions(extension_point_id)[:])
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/envisage/plugin.py", line 156, in get_extensions
    extensions = self._get_extensions_from_trait(trait_names[0])
  File "/Users/aayres/.edm/envs/envisage-test-3.6-pyqt5/lib/python3.6/site-packages/envisage/plugin.py", line 355, in _get_extensions_from_trait
    extensions = getattr(self, trait_name)
  File "envisage/examples/demo/plugins/tasks/attractors/attractors_plugin.py", line 47, in _tasks_default
    from attractors.visualize_2d_task import Visualize2dTask
  File "envisage/examples/demo/plugins/tasks/attractors/visualize_2d_task.py", line 10, in <module>
    from attractors.plot_2d_pane import Plot2dPane
  File "envisage/examples/demo/plugins/tasks/attractors/plot_2d_pane.py", line 2, in <module>
    from chaco.chaco_plot_editor import ChacoPlotItem
ModuleNotFoundError: No module named 'chaco.chaco_plot_editor'

since we removed chaco_plot_editor

EDIT: I've opened #409
These problems are probably better addressed in separate PRs, and this PR can be left small / easier to review

[rahulporuri]

LGTM.

[rahulporuri]

It's necessary to replace ChacoPlotItem here before the chaco major release. Im surprised I didn't find this when I searched Enthought org on GitHub for users. Searching again.