Make the group optional when building a tooltip

Currently, if a group is not set for a vertical bar chart's bar the
tooltip will display "undefined". This will ensure that if no group is
given the group will just be left out when building the tooltip.

Before:

![screen shot 2014-08-21 at 11 02 42 am](https://cloud.githubusercontent.com/assets/105694/3998015/82969fda-2944-11e4-9897-0f8125b04d39.png)


After:

![screen shot 2014-08-21 at 11 03 35 am](https://cloud.githubusercontent.com/assets/105694/3998017/86bdb1d4-2944-11e4-90c6-5c1ab5ab6e12.png)