Centering XAxis Labels to each integer point.

* [x ] 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 created a BarChartView using UIViewRepresentable. Then I created an value index mapper for xAxis like so:
`xAxis.valueFormatter = IndexAxisValueFormatter(values: xAxisLabels)`
xAxisLabels is just an array of strings.

## What did you expect to happen?

I expected the labels to be centered in each bar.

## What happened instead?

Instead the labels were drawn in a seemingly random position.

## Charts Environment

**Charts version/Branch/Commit Number:**
**Xcode version: Version 12.4 (12D4e)**
**Swift version: 5.3.3**
**Platform(s) running Charts: iPhone**
**macOS version running Xcode: 11.2.3**