Linechart zoom shows wrong preview

### Version
4.2.1-rc1

### Reproduction link
[https://jsfiddle.net/tmtron/0beLxwh2/21/](https://jsfiddle.net/tmtron/0beLxwh2/21/)

### Steps to reproduce

- add a null datapoint at 1995
- add some data between 1997 and 2000
- add a final null datapoint at 2005

### What is expected?
the zoom slider background data should show the real data in the middle of the slider

### What is actually happening?
The data in the zoom slider is stretched. I think this is a bug, unless I am missing some settings (but I could not find any switches on the zoom-slider to make this work)

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The following screenshot shows the issue. The area outside of the yellow highlighted rectangle in the zoom range (1) should not be visible.


see also: related [Stackoverflow question](https://stackoverflow.com/questions/55062223/linechart-zoom-shows-wrong-preview)