definition settings for specific chart types

As part of implementing the pie chart story it brings to mind the potential need to have a place in the definition where we can designate chart type specific configurations (inner-radius in terms of pie chart). Previously it lived in series, however that wont work if we have a multi-series pie chart for example. Thoughts @tomwayson ?

(Granted this could be accomplished via overrides)

Review the pie chart story and the existing handling of definitions, series, and overrides. Determine how chart-type-specific settings such as inner-radius should work for multi-series pie charts, then document the agreed design and its completion criteria; the issue currently has no named files or tests.

Written by the indexing model from the issue text.