Subchart inconsistency when using chart.flow function

C3 version: 0.4.10
D3 version: 3.5.6
Browser: Firefox 57.0.4
OS: OS X

https://codepen.io/anon/pen/jYexmM

Hi,

First of all thank you for this awesome library.
I tried to prototype one of the graphs I need to create and I run into an issue that my subchart looks inconsistent with the rest of the graph after data is added with `chart.flow` function. Also there is quite disturbing animation, but I'm mostly worried about an inconsistency.

Thanks for having a look and please let me know if I can be of any help.

Reproduce the inconsistency in the linked CodePen using C3 0.4.10, D3 3.5.6, and Firefox 57.0.4, focusing on the chart.flow call that adds data. Trace the related chart.flow entry point and determine what would make the subchart consistent with the rest of the graph; verify the result against the same example.

Written by the indexing model from the issue text.