handleCallback does not allow flush

Hi jettro,

I'm having trouble using the handleCallback, especially using the c3 API.
Basically, I want to redraw the legends (inset) of the pie chart after getting some data asynchronously.
I update the `pieChart` object, then call the `flush` method but it's not being updated.
I can define the legends only using the custom directives from c3-angular..

Here is a plunker that reproduce the issue:
https://plnkr.co/edit/9ZPGNg?p=preview

Anything I'm doing wrong?
Thx :)