Ginput giving wrong date for datetimeindex plot
I rewrote this [StackOverflow question](http://stackoverflow.com/questions/13832938/how-use-ginput-method-get-the-correct-points) to include an example. I think this looks like a bug (possibly in `num2date` or `date2num`)?

---

I read-in a file and plot it with pandas `DataFrame`. The index is DatetimeIndex, and then I use `ginput(1)` method to get one point, however the coordinate which I get is wrong.

The code is as follows:

```
import pandas as pd
from matplotlib.dates import num2date, date2num
ts = pd.date_range('2012-04-12,16:13:09', '2012-04-14,00:13:09', freq='H')
df = pd.DataFrame(index=ts)
df[0] = 20.6
```

I then plot and click on the graph using [ginput](http://glowingpython.blogspot.co.uk/2011/08/how-to-use-ginput.html):

```
df.plot()
t = pylab.ginput(n=1) #click somewhere near 13-APR-2012
```

However, the first item appears to be a float

```
In [8]: x = t[0][0] # ~ 370631.67741935479

In [9]: num2date(x)
Out[9]: datetime.datetime(1015, 10, 3, 16, 15, 29, 32253, tzinfo=<matplotlib.dates._UTC object at 0x104196550>)
# this is way out!
```

The docs suggest that it should be using these floats (from [`datetonum`](http://matplotlib.org/api/dates_api.html#matplotlib.dates.date2num)):

```
In [10]: dt = pd.to_datetime('13-4-2012', dayfirst=True)

In [11]: date2num(dt)
Out[11]: 734606.0
```

What is this float, and how can I convert it to a datetime?

_Note: If I remove one of the rows from the dataframe this works correctly:_

```
df1 = df.drop(ts[1], axis=0)
...
```
