Weird DatetimeIndex + secondary y axis plotting issue
Today I stumbled on weird issue with plotting two series with a datetimeindex and a secondary axis.

Code that illustrates the problem:

```python

a = pd.Series(
    [16, 13, 11],
    index=pd.to_datetime(['2017-09-13', '2017-09-14', '2017-09-16'], format='%Y-%m-%d')
)
b = pd.Series(
    [23, 27, 25],
    index=pd.to_datetime(['2017-09-13', '2017-09-14', '2017-09-15'], format='%Y-%m-%d')
)

# Combinations of `secondary_y` values to try out.
secondary_ys = [(False, False), (False, True), (True, False), (True, True)]

fig, axes = plt.subplots(figsize=(10, 2), ncols=4, nrows=1)
for (sya, syb), ax in zip(secondary_ys, axes.flat):
    a.plot(ax=ax, style='o-', secondary_y=sya)
    b.plot(ax=ax, style='x-', secondary_y=syb)
    ax.set_title('a:%r - b:%r' % (sya, syb))
```

- `b` is a straightforward time series with successive days
- `a` has a one day jump in the last item 
- the for loop tries out all the possible combinations of assigning these two series to the primary or secondary y axis in  a plot
- Whenever one or more series is assigned to the secondary y axis, the x axis is completely confused:
<img width="1100" alt="screen shot 2017-12-13 at 18 21 48" src="https://user-images.githubusercontent.com/44946/33952568-83c8ddc0-e032-11e7-91fd-2444f80eb2fa.png">


More in depth experimentation (and version information) can be found in the notebook at https://gist.github.com/soxofaan/9fdfdeafb8fb555dd8547bc48a27e2f3
(also notice the inconsistant x axes label formatting and orientation)
