Update c3 API:
Added 'c3chart.data.axes' function, it works similar to 'c3chart.data.colors' and it allows to change the data Y axes dynamically.
Add tooltip ungrouping for multiple X. Fix #628.
Add data.shown API and modify data API interface - #626
Fix ungrouped tooltip in multiple xs
Merge branch 'derek-schultz-ungroup-multiple-x'
Add data.axes API
Merge branch 'lbortoli-master'
Refactor data APIs and add spec
Fix tick text position when rotate
Rotate tick in redrawAxis
add support for array of tick label strings #139

formatted tick label can be an array of strings, each will be displayed
in a line
Calculate x axis height when tick rotated - #634
Fix interpolate when data id is a string that can be converted to a color - #604
Fix spec
Do not recompute padding left on showing tooltip - #637
Fix ticks after hide and show when axis.x.tick.fit = false - #638
Fix arc focus from legend event - #652
Fix selection when zoom enabled - #598
Apply clip path to subchart - #448
Fix mouse event for multiple xs - #663