How to use Charts to draw StreamGraph ?

[willychang21]

I want to draw a stream graph like this

I had already follow the LineChartFillDemo, and also saw the issue #4037 But it seems that the background line will still gone like this

but I want the rest of the streamgraph can have grid background.
this is what I'm done now.

this is what I'm done now

import UIKit
import Charts

class ViewController: UIViewController, ChartViewDelegate {
    
    @IBOutlet var chartView: LineChartView!

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        self.title = "Filled Line Chart"
        
        chartView.delegate = self
        
        // chartView.backgroundColor = .white
        chartView.gridBackgroundColor = .white // UIColor(red: 51/255,
        // green: 181/255,
        // blue: 229/255,
        // alpha: 150/255)
        chartView.drawGridBackgroundEnabled = true
        
        chartView.drawBordersEnabled = true
        
        chartView.chartDescription.enabled = false
        
        chartView.pinchZoomEnabled = false
        chartView.dragEnabled = true
        chartView.setScaleEnabled(true)
        
        chartView.legend.enabled = false
        
        chartView.xAxis.enabled = false
        
        let leftAxis = chartView.leftAxis
        leftAxis.axisMaximum = 900
        leftAxis.axisMinimum = -250
        leftAxis.drawAxisLineEnabled = false
        
        chartView.rightAxis.enabled = false
  
        setDataCount(Int(30), range: UInt32(30))
    }
    
    func setDataCount(_ count: Int, range: UInt32) {
        let yVals1 = (0..<count).map { (i) -> ChartDataEntry in
            let val = Double(arc4random_uniform(range) + 50)
            return ChartDataEntry(x: Double(i), y: val)
        }
        let yVals2 = (0..<count).map { (i) -> ChartDataEntry in
            let val = Double(arc4random_uniform(range) + 450)
            return ChartDataEntry(x: Double(i), y: val)
        }
        let yVals3 = (0..<count).map { (i) -> ChartDataEntry in
            let val = Double(arc4random_uniform(range) + 600)
            return ChartDataEntry(x: Double(i), y: val)
        }
        
        let set1 = LineChartDataSet(entries: yVals1, label: "DataSet 1")
        set1.axisDependency = .left
        set1.setColor(UIColor(red: 255/255, green: 241/255, blue: 46/255, alpha: 1))
        set1.drawCirclesEnabled = false
        set1.lineWidth = 2
        set1.circleRadius = 3
        set1.fillAlpha = 1
        set1.drawFilledEnabled = true
        set1.fillColor = .red
        set1.highlightColor = UIColor(red: 255/255, green: 117/255, blue: 117/255, alpha: 1)
        set1.drawCircleHoleEnabled = false
        set1.fillFormatter = DefaultFillFormatter { _,_  -> CGFloat in
            return CGFloat(self.chartView.leftAxis.axisMaximum)
        }
        
        let set2 = LineChartDataSet(entries: yVals2, label: "DataSet 2")
        set2.axisDependency = .left
        set2.setColor(UIColor(red: 255/255, green: 241/255, blue: 46/255, alpha: 1))
        set2.drawCirclesEnabled = false
        set2.lineWidth = 2
        set2.circleRadius = 3
        set2.fillAlpha = 1
        set2.drawFilledEnabled = true
        set2.fillColor = .green
        set2.highlightColor = UIColor(red: 244/255, green: 117/255, blue: 117/255, alpha: 1)
        set2.drawCircleHoleEnabled = false
        set2.fillFormatter = DefaultFillFormatter { _,_  -> CGFloat in
            return CGFloat(self.chartView.leftAxis.axisMaximum)
        }
        
        let set3 = LineChartDataSet(entries: yVals3, label: "DataSet 3")
        set3.axisDependency = .left
        set3.setColor(UIColor(red: 255/255, green: 241/255, blue: 46/255, alpha: 1))
        set3.drawCirclesEnabled = false
        set3.lineWidth = 2
        set3.circleRadius = 3
        set3.fillAlpha = 1
        set3.drawFilledEnabled = true
        set3.fillColor = .white
        set3.highlightColor = UIColor(red: 244/255, green: 117/255, blue: 117/255, alpha: 1)
        set3.drawCircleHoleEnabled = false
        set3.fillFormatter = DefaultFillFormatter { _,_  -> CGFloat in
            return CGFloat(self.chartView.leftAxis.axisMaximum)
        }

        let data: LineChartData = [set1, set2, set3]
        data.setDrawValues(false)
        
        chartView.data = data
    }
    

}

and it showed like this

the line above green area was disappear because I use white cover it, and I dont know whether have better alternative way to do it.

Actually I want to change the fillformater, but each set only can setup one fillformatter.