[FIX](VisualRoles Null Required) A visual role specified with value null, to prevent default binding, would not validate its requiredness.
[FEAT](ColorAxis _Unbound_ option) Added new option that is used for controlling the color that is shown when an optional color role is not bound. By default it takes the first color of axis option 'Colors'.
[CHG](BaseChart#visualRoles) Changed method to a plain object. Now panels and charts use the same interface.