Combined chart with different entries count for line chart and bar chart

* [ ok] I've read, understood, and done my best to follow the [*CONTRIBUTING guidelines](https://github.com/jjatie/Charts/blob/master/CONTRIBUTING.md).

## What did you do?
I am trying to make combined chart with bar and line chart. I want to create fixed visible range from current time and one hour from there. So as time passes more bars appear on that fixed one hour timeline. for bar chart its easy, i just added missing values. but when i do that linechart disapears. If i add missing value to linechart aswell it looks awfull, because it drops to zero and i dont want that. Is it possible to have different entries count on those line and bar chart. Or does anybody have a suggestion on how to implement fixed range on combined chart so linechart doesnt drop to zero. i just want line to end depending on current time in timeline.

No files, tests, or entry points are named. Start by reproducing the combined bar-and-line chart with a fixed one-hour visible range, then inspect the combined chart data-entry handling. Done means the bar series can fill the timeline while the line series ends at the current time without dropping to zero.

Written by the indexing model from the issue text.