Plotting methods (line, area, ..) don't handle string x values
From https://github.com/pandas-dev/pandas/pull/22307#pullrequestreview-145585750: when specifying the x labels in one of our plotting calls where this consists of strings, nothing is shown on the x labels:

```
In [6]: df = pd.DataFrame({'sales': [3, 2, 3],
   ...:                    'visits': [20, 42, 28],
   ...:                    'day': ['Monday', 'Tuesday', 'Wednesday']})

In [7]: ax = df.plot.area(x='day')
```

![figure_1-1](https://user-images.githubusercontent.com/1020496/44081733-b7cb56ba-9faf-11e8-89f0-e38544760df2.png)

The same is true for line plot, for bar plot it does work.

Related to https://github.com/pandas-dev/pandas/issues/18687 (but not exactly the same), and potentially https://github.com/pandas-dev/pandas/pull/18726. I would have expected that there is already an issue for this, but didn't directly find it.