BUG: fix variable overwriting in radviz
The variable `n` is set to number of rows in dataframe, and then it overwritten with number of columns, but after that there is loop that should iterate over rows and it uses new value in `n` that holds number of columns already.

can u add a test for this
(that validates the returned axes / graph)
 @TomAugspurger ?
 Yep, good catch. @stas-sl do you mind rebasing and adding a note in `docs/source/v0.15.0.txt`?

And if you want, a test would be great. radviz doesn't seem to be that thoroughly tested.
 Did rebase and added the note, but sorry my knowledge of python, pandas and running tests in python is not sufficient, so currently can't help you with tests.
 Yeah I looked at it a bit and didn't see an obvious way to check.
 Thanks @stas-sl!
 The docs are built automatically during the Travis run, would you mind checking back [here](http://pandas-docs.github.io/pandas-docs-travis/visualization.html#radviz) later today to see if the radviz plot looks correct?
 It looks much better now )

Before:
![before](https://cloud.githubusercontent.com/assets/4602302/4327623/03f06614-3f7b-11e4-9fad-0081a13b06a9.png)

After:
![after](https://cloud.githubusercontent.com/assets/4602302/4327622/03ee48a2-3f7b-11e4-9d76-2e194d53ffff.png)
 Thanks!
