Subchart resets zoom when `chart.subchart.show()` is called

Hi All,
the function `subchart.show()` seems to reset the current zoom to the full domain.

Here's an attached jfiddle reproducing the issue: https://jsfiddle.net/a1t82cyn/3/

* **C3 version**: 0.7.15
* **D3 version**: 5.16.0
* **Browser**: Chrome 83.0.4103.61, Firefox 77.0.1
* **OS**: Windows 10

I think it might be related to the problem mentioned in this issue (still valid in version 0.7.15): https://github.com/c3js/c3/issues/2613