Barchart space between vertical bars (yVals) for stacked bars

* [ ] 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?

How do I add spacing / padding / separator between stacked vertical bars? i.e. spacing between yVals.

## What did you expect to happen?

This is what I would like to see.


## What happened instead?

This is what I see with current dataset.


## Charts Environment

**Charts version/Branch/Commit Number:**
**Xcode version:**
**Swift version:**
**Platform(s) running Charts:**
**macOS version running Xcode:**

## Demo Project