LineChart Dataset not rendering

* [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?

Attempting to draw to line chart datasets.

## What did you expect to happen?

Both datasets should be rendered.

## What happened instead?

Only one dataset was rendered.

## Charts Environment

**Charts version/Branch/Commit Number:** master branch
**Xcode version:** 13.0 beta 3
**Swift version:** 5.x
**Platform(s) running Charts:** macOS
**macOS version running Xcode:** 12.0 beta 3

## Reason

Tracked the issue down to the following method.

open override func entryIndex(
x xValue: Double,
closestToY yValue: Double,
rounding: ChartDataSetRounding) -> Int

in

ChartDataSet

is returning -1 instead of the last index when xValue is equal to or larger than the x-axis entries. This led to an empty range being drawn for one of the two datasets.