0.10.1: DataFrame.plot(x='x', y='y') does not correctly plot one column versus another
```
df = DataFrame(index=range(100))
df['x'] = np.random.randn(100)
df['y'] = df['x']
df.plot(x='x', y='y', linestyle='None', marker='o')
```

I expected to see column 'y' plotted against column 'x' and, as such, to see scatter points along the 45-degree line. Instead, I get a strange scatter plot. Am I doing something wrong or is this a bug? If it is a bug, is there a workaround?
