Ref T13279. Currently, we store a fairly low-level description of functions and datasets in a chart. This will create problems with (for example) translating function labels.
If you view a chart someone links you, it should say "El Charto" if you speak Spanish, not "The Chart" if the original viewer speaks English.
To support this, store a slightly higher level version of the chart: the chart engine key, plus configuration parameters. This is very similar to how SearchEngine works.
For example, the burndown chart now stores a list of project PHIDs, instead of a list of [accumulate [sum [fact task.open <project-phid>]]] functions.
(This leaves some serialization code with no callsites, but we may eventually have a "CustomChartEngine" which stores raw functions, so I'm leaving it for now.)
As a result, function labels provided by the chart engine are now translatable.
(Note that the actual chart is meaningless since the underlying facts can't be stacked like they're being stacked, as some are negative in some areas of their accumulation.)