ENH: plotting numerical expressions
Passing the `y` argument of a plot to `DataFrame.eval` allows plotting a numerical expression.
Makes numerical data exploration much easier. E.g. graphically finding the norm of two curves.
See example here:
[example notebook](https://gist.github.com/TsvikiHirsh/46fd47bc35dc1874234d)

No support for multicolumn plotting, e.g. for `["a","4*b"]`
Should be very easy to apply the same on the `index`.

Could be very useful for data exploration, without modifying the original `DataFrame`
