What's the real purpose and reason of having BarChart<X, Y> when Category axis is a final class that extends Axis<String>?

This forces bar charts to have one of their axis that has String as its type (ie: either BarChart<String, Y> or BarChart<X, String>)
There should be no real reason why CategoryAxis requires to use String as a tuype, a category could be any type that can be sorted and have a unique hashValue (ie: Integer, others)...

CategoryAxis extends Axis<String> should become CategoryAxis<T> extends Axis<T>.
It should also feature the same label formatter utility that ValueAxis has but tailored to the generic "T" type (instead of a "T extends Number" type). Default formatter could call String.valueOf() or T.toString().