PCP - summary stats for selected/unselected, test on difference in means

```
Similar to the logic used for the scatter plot, we could implement a test on
difference in means (a simple z-stat) between selected and unselected
observations in PCP, for each of the variables included.

A related idea is to visualize the summary statistics that are now given, i.e.,
show a line that corresponds to the mean values for each variable.

```

Original issue reported on code.google.com by `lanse...@gmail.com` on 11 Sep 2013 at 4:46