Improve PCP display

```
from Aug 18, 2012

One of the problems with the current PCP display for larger datasets is that
larger values are drawn on top of smaller ones, giving a distorted visual
impression. One solution Mark suggested to try instead (or as an additional
display option) is to have the color scheme of the lines be based on a random
sample to give a more representative visual impression of the values in the
dataset.

```

Original issue reported on code.google.com by `mmcc...@gmail.com` on 5 Mar 2013 at 6:24