LineChartDataSet not showing one of the lines

* [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?

Tried to create a step chart using the line graph. I wanted to show two separate lines.

## What did you expect to happen?
I expected this to work pretty straightforward and show both lines.

## What happened instead?

One of the lines will not show unless every point is on the same x-axis. I will have to zoom in and out to sometimes see one of the lines. Occasionally, when zooming, it will crash saying `lowerbound <= upperbound`, but when debugging it i can't make sense of it.

This video shows how it will initially look and then what I have to do to show the lines.

https://user-images.githubusercontent.com/83983169/122051453-3b6bff80-cdaa-11eb-9169-e8b834702916.mp4

## Charts Environment

**Charts version/Branch/Commit Number: 4.0.1**
**Xcode version: 12.5**
**Swift version:5**
**Platform(s) running Charts: iOS**
**macOS version running Xcode: 11.3.1**

## Demo Project

ℹ Please link to or upload a project we can download that reproduces the issue.

https://github.com/sahilreddyifit/ChartExample

Start by downloading the linked ChartExample project and reproducing the issue with Charts 4.0.1 on iOS. Trace how the LineChartDataSet is configured for the step chart and how zooming affects the two lines, then verify that both lines render initially and that zooming does not trigger the reported lowerbound <= upperbound crash.

Written by the indexing model from the issue text.