Linear Gradient Fill not updating when viewport changes (Chart zoom in/out or pan)

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## What did you do?

Added a Filled horizontal linear gradient to a LineChart's dataSet then zoomed in and panned the view.
```swift
// I use swiftui but here's a general idea of the config I've given the chart
let lineChartView = LineChartView()

let chartDataSet = LineChartDataSet( entries: [
ChartDataEntry(x: 1.0, y: 5.0),
ChartDataEntry(x: 2.0, y: 10.0),
ChartDataEntry(x: 3.0, y: 20.0),
ChartDataEntry(x: 4.0, y: 15.0),
ChartDataEntry(x: 5.0, y: 0.0),
])

chartDataSet.drawCirclesEnabled = false
chartDataSet.drawCircleHoleEnabled = false
chartDataSet.drawFilledEnabled = true
chartDataSet.drawHorizontalHighlightIndicatorEnabled = false
chartDataSet.drawIconsEnabled = false
chartDataSet.drawVerticalHighlightIndicatorEnabled = false
chartDataSet.fillAlpha = 255
chartDataSet.highlightEnabled = false
chartDataSet.lineWidth = 0
chartDataSet.valueFont = .systemFont(ofSize: 9)

let gradient = CGGradient(
colorsSpace: nil,
colors: [UIColor.red.cgColor, UIColor.red.cgColor, UIColor.blue.cgColor, UIColor.blue.cgColor, UIColor.green.cgColor] as CFArray,
locations: [0.0, 0.3, 0.3, 0.6, 0.6]
)
chartDataSet.fill = Fill.fillWithLinearGradient(gradient!, angle: 0.0)

lineChartView.data = LineChartData(dataSet: chartDataSet)
lineChartView.dragYEnabled = false
lineChartView.legend.enabled = false
lineChartView.rightAxis.enabled = false
lineChartView.scaleYEnabled = false
lineChartView.xAxis.labelPosition = .bottom
```

## What did you expect to happen?

I expected the linear gradient to stretch and shrink with the change of scale for the Line

## What happened instead?

The linear gradient ratios are stuck to the bounds of the chart, not the dataset

`GIF Below`


## Charts Environment

**Charts version/Branch/Commit Number:** 3.6
**Xcode version:** 12.1
**Swift version:** 5
**Platform(s) running Charts:**
iOS 13.5, iOS: 14.1
**macOS version running Xcode:**

## Demo Project

ℹ Please link to or upload a project we can download that reproduces the issue.
TBA (I can add one later in my spare time with storyboard if requested)