Draw Graph curves with UIBezierPath

Using @user1244109's answer I implement a better algorithm in Swift 5.

class GraphView: UIView {

    var data: [CGFloat] = [2, 6, 12, 4, 5, 7, 5, 6, 6, 3] {
        didSet {
            setNeedsDisplay()
        }
    }

    func coordYFor(index: Int) -> CGFloat {
        return bounds.height - bounds.height * data[index] / (data.max() ?? 0)
    }

    override func draw(_ rect: CGRect) {

        let path = quadCurvedPath()

        UIColor.black.setStroke()
        path.lineWidth = 1
        path.stroke()
    }

    func quadCurvedPath() -> UIBezierPath {
        let path = UIBezierPath()
        let step = bounds.width / CGFloat(data.count - 1)

        var p1 = CGPoint(x: 0, y: coordYFor(index: 0))
        path.move(to: p1)

        drawPoint(point: p1, color: UIColor.red, radius: 3)

        if (data.count == 2) {
            path.addLine(to: CGPoint(x: step, y: coordYFor(index: 1)))
            return path
        }

        var oldControlP: CGPoint?

        for i in 1..<data.count {
            let p2 = CGPoint(x: step * CGFloat(i), y: coordYFor(index: i))
            drawPoint(point: p2, color: UIColor.red, radius: 3)
            var p3: CGPoint?
            if i < data.count - 1 {
                p3 = CGPoint(x: step * CGFloat(i + 1), y: coordYFor(index: i + 1))
            }

            let newControlP = controlPointForPoints(p1: p1, p2: p2, next: p3)

            path.addCurve(to: p2, controlPoint1: oldControlP ?? p1, controlPoint2: newControlP ?? p2)

            p1 = p2
            oldControlP = antipodalFor(point: newControlP, center: p2)
        }
        return path;
    }

    /// located on the opposite side from the center point
    func antipodalFor(point: CGPoint?, center: CGPoint?) -> CGPoint? {
        guard let p1 = point, let center = center else {
            return nil
        }
        let newX = 2 * center.x - p1.x
        let diffY = abs(p1.y - center.y)
        let newY = center.y + diffY * (p1.y < center.y ? 1 : -1)

        return CGPoint(x: newX, y: newY)
    }

    /// halfway of two points
    func midPointForPoints(p1: CGPoint, p2: CGPoint) -> CGPoint {
        return CGPoint(x: (p1.x + p2.x) / 2, y: (p1.y + p2.y) / 2);
    }

    /// Find controlPoint2 for addCurve
    /// - Parameters:
    ///   - p1: first point of curve
    ///   - p2: second point of curve whose control point we are looking for
    ///   - next: predicted next point which will use antipodal control point for finded
    func controlPointForPoints(p1: CGPoint, p2: CGPoint, next p3: CGPoint?) -> CGPoint? {
        guard let p3 = p3 else {
            return nil
        }

        let leftMidPoint  = midPointForPoints(p1: p1, p2: p2)
        let rightMidPoint = midPointForPoints(p1: p2, p2: p3)

        var controlPoint = midPointForPoints(p1: leftMidPoint, p2: antipodalFor(point: rightMidPoint, center: p2)!)

        if p1.y.between(a: p2.y, b: controlPoint.y) {
            controlPoint.y = p1.y
        } else if p2.y.between(a: p1.y, b: controlPoint.y) {
            controlPoint.y = p2.y
        }


        let imaginContol = antipodalFor(point: controlPoint, center: p2)!
        if p2.y.between(a: p3.y, b: imaginContol.y) {
            controlPoint.y = p2.y
        }
        if p3.y.between(a: p2.y, b: imaginContol.y) {
            let diffY = abs(p2.y - p3.y)
            controlPoint.y = p2.y + diffY * (p3.y < p2.y ? 1 : -1)
        }

        // make lines easier
        controlPoint.x += (p2.x - p1.x) * 0.1

        return controlPoint
    }

    func drawPoint(point: CGPoint, color: UIColor, radius: CGFloat) {
        let ovalPath = UIBezierPath(ovalIn: CGRect(x: point.x - radius, y: point.y - radius, width: radius * 2, height: radius * 2))
        color.setFill()
        ovalPath.fill()
    }

}

extension CGFloat {
    func between(a: CGFloat, b: CGFloat) -> Bool {
        return self >= Swift.min(a, b) && self <= Swift.max(a, b)
    }
}

Expand on Abid Hussain's answer on Dec 5 '12.

i implemeted the code, and it worked but result looked like that:

With little adjustment, i was able to get what i wanted:

What i did was addQuadCurveToPoint to mid points as well, using mid-mid points as control points. Below is the code based on Abid's sample; hope it helps someone.

+ (UIBezierPath *)quadCurvedPathWithPoints:(NSArray *)points
{
    UIBezierPath *path = [UIBezierPath bezierPath];

    NSValue *value = points[0];
    CGPoint p1 = [value CGPointValue];
    [path moveToPoint:p1];

    if (points.count == 2) {
        value = points[1];
        CGPoint p2 = [value CGPointValue];
        [path addLineToPoint:p2];
        return path;
    }

    for (NSUInteger i = 1; i < points.count; i++) {
        value = points[i];
        CGPoint p2 = [value CGPointValue];

        CGPoint midPoint = midPointForPoints(p1, p2);
        [path addQuadCurveToPoint:midPoint controlPoint:controlPointForPoints(midPoint, p1)];
        [path addQuadCurveToPoint:p2 controlPoint:controlPointForPoints(midPoint, p2)];

        p1 = p2;
    }
    return path;
}

static CGPoint midPointForPoints(CGPoint p1, CGPoint p2) {
    return CGPointMake((p1.x + p2.x) / 2, (p1.y + p2.y) / 2);
}

static CGPoint controlPointForPoints(CGPoint p1, CGPoint p2) {
    CGPoint controlPoint = midPointForPoints(p1, p2);
    CGFloat diffY = abs(p2.y - controlPoint.y);

    if (p1.y < p2.y)
        controlPoint.y += diffY;
    else if (p1.y > p2.y)
        controlPoint.y -= diffY;

    return controlPoint;
}