Tsplot gaps
Issue #1466

I don't think this fix works for irregular time series:

```
In [2]: rng = date_range('4/16/2012', periods=1000, freq='h')

In [3]: rng = rng[rng.hour < 7]

In [4]: ts = Series(np.random.randn(len(rng)).cumsum(), rng)

In [5]: ts.plot()
Out[5]: <matplotlib.axes.AxesSubplot at 0x2a63050>
```
 This is great, thanks
