Limit the min zoom out factor along x axis to prevent date range error

[midhun-pm]

Fixes part of issue 415

[rahulporuri]

While this change fixes the possibility of getting the exception when zooming out, I can still get the error if I

1. zoom out to the max extent
2. start scrolling towards the left.

/Users/rporuri/.edm/envs/chaco-test-3.6-pyqt/lib/python3.6/site-packages/chaco/scales/safetime.py:37: UserWarning: Timestamp out of range.  Returning safe default value.
  warnings.warn("Timestamp out of range.  Returning safe default value.")
Traceback (most recent call last):
  File "/Users/rporuri/.edm/envs/chaco-test-3.6-pyqt/lib/python3.6/site-packages/enable/qt4/base_window.py", line 213, in paintEvent
    self.handler.paintEvent(event)
  File "/Users/rporuri/.edm/envs/chaco-test-3.6-pyqt/lib/python3.6/site-packages/enable/qt4/base_window.py", line 65, in paintEvent
    self._enable_window._paint(event)
  File "/Users/rporuri/.edm/envs/chaco-test-3.6-pyqt/lib/python3.6/site-packages/enable/abstract_window.py", line 471, in _paint
    self.component.draw(gc, view_bounds=(0, 0, size[0], size[1]))
  File "/Users/rporuri/.edm/envs/chaco-test-3.6-pyqt/lib/python3.6/site-packages/enable/component.py", line 427, in draw
    self._draw(gc, view_bounds, mode)
  File "/Users/rporuri/.edm/envs/chaco-test-3.6-pyqt/lib/python3.6/site-packages/enable/component.py", line 779, in _draw
    self._dispatch_draw(layer, gc, view_bounds, mode)
  File "/Users/rporuri/.edm/envs/chaco-test-3.6-pyqt/lib/python3.6/site-packages/enable/container.py", line 273, in _dispatch_draw
    component._dispatch_draw(layer, gc, new_bounds, mode)
  File "/Users/rporuri/.edm/envs/chaco-test-3.6-pyqt/lib/python3.6/site-packages/enable/component.py", line 799, in _dispatch_draw
    handler(gc, view_bounds, mode)
  File "/Users/rporuri/.edm/envs/chaco-test-3.6-pyqt/lib/python3.6/site-packages/enable/component.py", line 887, in _draw_underlay
    underlay.overlay(self, gc, view_bounds, mode)
  File "/Users/rporuri/.edm/envs/chaco-test-3.6-pyqt/lib/python3.6/site-packages/chaco/grid.py", line 307, in overlay
    self._compute_ticks(other_component)
  File "/Users/rporuri/.edm/envs/chaco-test-3.6-pyqt/lib/python3.6/site-packages/chaco/grid.py", line 255, in _compute_ticks
    scale=scale)
  File "/Users/rporuri/.edm/envs/chaco-test-3.6-pyqt/lib/python3.6/site-packages/chaco/scales_tick_generator.py", line 31, in get_ticks
    ticks = self.scale.ticks(data_low, data_high)
  File "/Users/rporuri/.edm/envs/chaco-test-3.6-pyqt/lib/python3.6/site-packages/chaco/scales/scales.py", line 446, in ticks
    ticks = scale.ticks(start, end, numticks)
  File "/Users/rporuri/.edm/envs/chaco-test-3.6-pyqt/lib/python3.6/site-packages/chaco/scales/time_scale.py", line 291, in ticks
    return trange(start, end, **{self.unit: self.val})
  File "/Users/rporuri/.edm/envs/chaco-test-3.6-pyqt/lib/python3.6/site-packages/chaco/scales/time_scale.py", line 179, in trange
    return trange_years(start, end, value)
  File "/Users/rporuri/.edm/envs/chaco-test-3.6-pyqt/lib/python3.6/site-packages/chaco/scales/time_scale.py", line 137, in trange_years
    dt = datetime(dt_start.year-dyears, 1, 1, 0, 0, 0, 0)
ValueError: year 0 is out of range

[midhun-pm]

While this change fixes the possibility of getting the exception when zooming out, I can still get the error if I

1. zoom out to the max extent

2. start scrolling towards the left.

Thanks for catching this, didn't notice it.

Fixed this by restricting the pan tool to available data along X

[shoeb-github]

From #415, the demo breaks for these files too.

The following demos also break when the user zooms out too much
shell/dates.py
financial/correlations.py
financial/stock_prices.py

[corranwebster]

Looks good, but only fixes one of the deoms listed in #415 - don't mark #415 as "fixed" in the description.