Add class descriptions.  These two classes' roles were not very clear.

Change-Id: Ib45c9037f2c4b3ee5bfed79f3c93e5020bac477b
Tuck this anonymous namespace inside chart namespace.

This way we can eliminate those explicit '::chart' qualifications.

Change-Id: I764e0ca57c8e10f7eac47f48a4cabf28d0562113