Fix single-shot timers in nbagg backend

[joferkington]

Single-shot timers are currently non-functional (raise an error) when using the nbagg backed.

To reproduce the issue, try running the following in an ipython notebook (I can't seem to find a way to set up a fully independent nbagg figure without manually starting a "full" notebook):

import numpy as np
import matplotlib
matplotlib.use('nbAgg')
import matplotlib.pyplot as plt

fig, ax = plt.subplots()

def callback(ax):
    ax.plot(np.random.random((10, 10)))
    ax.figure.canvas.draw()

timer = fig.canvas.new_timer(1000, [(callback, [ax], {})])
timer.single_shot = True
timer.start()

plt.show()

Ideally, you'd get a figure that would update with a plot 1 second after being displayed. Instead, you'll get an AttributeError:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-1-346b07a4494a> in <module>()
     13 timer = fig.canvas.new_timer(500, [(callback, [ax], {})])
     14 timer.single_shot = True
---> 15 timer.start()
     16 #timer.stop()
     17 

/home/jofer/anaconda/lib/python2.7/site-packages/matplotlib/backend_bases.pyc in start(self, interval)
   1218         if interval is not None:
   1219             self._set_interval(interval)
-> 1220         self._timer_start()
   1221 
   1222     def stop(self):

/home/jofer/anaconda/lib/python2.7/site-packages/matplotlib/backends/backend_nbagg.pyc in _timer_start(self)
    186                 self._on_timer,
    187                 self.interval)
--> 188         self._timer.start()
    189 
    190     def _timer_stop(self):

AttributeError: '_Timeout' object has no attribute 'start'

Fully fixing this issue is slightly more complex than it seems at first glance. Basically, single-shot timers need to follow a completely different code path than regular timers when using Tornado.

I've added some additional user acceptance tests to /lib/matplotlib/backends/web_backend/nbagg_uat.ipynb and can vouch that the current changes don't break any of the manual user acceptance tests in the notebook.

[tacaswell]

The single-shot time on UAT 17 runs indefinitely for me

[tacaswell]

should be timer

[joferkington]

Woops! I'd fixed that in the notebook I was running but forgot to save it before committing. Thanks for catching that!

[joferkington]

@tacaswell - The typo-fix in UAT17 should fix the indefinitely-running problem. I'll verify it tonight when I can test it (at work now). Thanks for catching that!

[tacaswell]

@joferkington Thanks!