Plot expressions
It would be nice to have the possibility to plot expressions. Suppose I have a dataframe with three columns: col1, col2, col3. Now I can do:

```
df.plot.scatter(x='col1', y='col2')
```

it would be nice to be able to plot expressions directly:

```
df.plot.scatter(x='col1 ** 2', y = 'col2 / sqrt(col3)')
```

and also for other plotting methods (e.g. histograms)
