Feature request: Ability to retain consistent y axis values when hiding/showing chart data

* **C3 version**: 0.4.18
* **D3 version**: 1.0.8
* **Browser**: Chrome 63.0.3239.132
* **OS**: OSX 10.13.2

When hiding or showing different datasets in a stacked area chart, the y axis automatically updates to rescale the y axis based on the new maximum values. Is it possible to pass a set of values to `axis.y.ticks.values` that are then kept consistent regardless of which datasets are currently being rendered? Currently these seem to be overwritten once the datasets hidden/shown are toggled.

Alternatively, passing an option to "stick" the y axis values at whatever they are when the chart first renders would be helpful.

If there's already a way of doing this then any help would be much appreciated