Introduce ChartItems instead of Json API

Different nvd3 charts require different JSON formats. Don't let the
application developer deal with these.

Provide a simple data model for chart data, with immutable item objects
that can be shared between sessions. These objects don't have a remote
counterpart, they're rendered as JSON on setItems().

Change-Id: I48bc7f5239698ea2b0c1634c900846233d631ad8