CandleChart.swift

import UIKit
import PlaygroundSupport

let chartData: [Double] = [1, 2, 4, 0, 0, 2, 6, 0, 4, 8, 12, 14, 1, 2, 2, 0, 0, 1, 6, 0, 4, 0, 3, 0]

class TimelineCandleChart: UIView {

    var data: [Double] = [] {
        didSet { setNeedsDisplay() }
    }

    var candleTopColor = UIColor.cyan {
        didSet { setNeedsDisplay() }
    }

    var candleBottomColor = UIColor.blue {
        didSet { setNeedsDisplay() }
    }

    var candleMargin: CGFloat = 5 {
        didSet { setNeedsDisplay() }
    }

    override func draw(_ rect: CGRect) {
        guard data.count > 0 else { return }
        guard let ctx = UIGraphicsGetCurrentContext() else { return }

        let candleWidth: CGFloat = bounds.width / CGFloat(data.count) - candleMargin

        let gradient = CGGradient(colorsSpace: nil, colors: [candleTopColor.cgColor, candleBottomColor.cgColor] as CFArray, locations: [0, 1])!

        let max = data.reduce(0) { $0 < $1 ? $1 : $0 }

        for i in 0..<data.count {
            ctx.saveGState()

            let x = (candleWidth + candleMargin) * CGFloat(i)
            let height = CGFloat(data[i]) / CGFloat(max) * bounds.height

            let minHeight = CGFloat.maximum(candleWidth, height)

            let candleRect = CGRect(x: x, y: bounds.height - minHeight, width: candleWidth, height: minHeight)
            let candlePath = UIBezierPath(roundedRect: candleRect, cornerRadius: candleWidth / 2)

            ctx.addPath(candlePath.cgPath)

            ctx.clip()

            ctx.beginTransparencyLayer(auxiliaryInfo: nil)
            ctx.drawLinearGradient(gradient, start: CGPoint(x: 0, y: 0), end: CGPoint(x: 0, y: bounds.height), options: [.drawsBeforeStartLocation, .drawsAfterEndLocation])
            ctx.endTransparencyLayer()

            ctx.restoreGState()
        }

    }
}

let chart = TimelineCandleChart()
chart.frame = CGRect(x: 0, y: 0, width: 400, height: 300)
chart.data = chartData

PlaygroundPage.current.liveView = chart