Possible bug in DataFrame.plot when duplicates in DatetimeIndex
There seems to be a bug when plotting dataframes that has duplicates in a DatetimeIndex. It seems to only be triggered when there is at least one more non-duplicate item in the index as in the first two cases below. These result in a:

``` python
ValueError: Shape of passed values is (1, 3), indices imply (1, 2)
```

While the last case seems to be fine.

``` python

    # Case 1: fails 
    df = pandas.DataFrame(
        index = pandas.to_datetime([
            "2002-07-18 13:49:49",
            "2002-07-18 13:49:49",
            "2002-07-18 10:38:58"
        ]),
        data = [1, 1, 1]
    )
    df.plot()

    # Case 2: fails
    df = pandas.DataFrame(
        index = pandas.to_datetime([
            "2002-07-19 10:51:29",
            "2002-07-18 13:49:49",
            "2002-07-18 13:49:49",
        ]),
        data = [1, 1, 1]
    )
    df.plot()

    # Case 3: works
    df = pandas.DataFrame(
        index = pandas.to_datetime([
            "2002-07-18 13:49:49",
            "2002-07-18 13:49:49",
        ]),
        data = [1, 1]
    )
    df.plot()

```

This is with a recent master checkout: 52a82efe98a52df7a2dbb1208df19622ccdff39c
