Use static type annotations for Chart and Tracestyle methods and extensions

[kMutagene]

Related: #70

While leaving an optional parameter as 'A in F# works just fine, specific annotations for these parameters are mandatory in C# (see linked issue), making the Chart API hard to use in those cases where no type annotation is set in the source code.

Static type annotations solve this problem and should be set for the main API at least.

[kMutagene]

This is the standard in the code base now.