Currently Fill relies on an enum to determine which fill it is going to use. Since enums cannot have cases added to them, consumers cannot extend the FillType to add new types of chart fills. The current fillPath method would also need to be changed.

To fix this I propose creating a protocol all Fill types must adhere to as implemented in #3084. This allows users to create any Fill they desire in their apps, and don't require changes to be made to the framework. This also removes any confusion of unused variables as the current implementation requires all possible variables for every type in the same class.