[API Review]: RT-28148 - Add createSymbols Property to AreaChart / StackedAreaChart

[Richard Bair]

> The other problem is that the generics around the Callback make it
> virtually unreadable.  If you want to extend Callback to provide a more
> meaningful interface, that might work.

Just real quickly on this point, with Lambda's this should all melt away. Lambda's bring with them some additional type inference which will be able (in most cases, and I think in all cases with Callback) to remove all the noise. So it becomes:

foo.setOnWhateverCallback((arg) -> { something });

And that's it.