How to create a floating graph marker with ios-charts

Question

I\'m using the ios-charts framework and want to create a marker that floats over the graph when I touch and move my finger from side to side. I\'m using a line chart just for r

Answer 1

This is based on the example BallonMarker provided at the iOS-Charts repo.

Hopefully this comes in handy for someone, as I couldn't find other subclass examples of MarkerImage so this was a lot of trail and error for me as never used CGContext before

To use add this class

class PillMarker: MarkerImage {

    private (set) var color: UIColor
    private (set) var font: UIFont
    private (set) var textColor: UIColor
    private var labelText: String = ""
    private var attrs: [NSAttributedString.Key: AnyObject]!

    static let formatter: DateComponentsFormatter = {
        let f = DateComponentsFormatter()
        f.allowedUnits = [.minute, .second]
        f.unitsStyle = .short
        return f
    }()

    init(color: UIColor, font: UIFont, textColor: UIColor) {
        self.color = color
        self.font = font
        self.textColor = textColor

        let paragraphStyle = NSMutableParagraphStyle()
        paragraphStyle.alignment = .center
        attrs = [.font: font, .paragraphStyle: paragraphStyle, .foregroundColor: textColor, .baselineOffset: NSNumber(value: -4)]
        super.init()
    }

    override func draw(context: CGContext, point: CGPoint) {
        // custom padding around text
        let labelWidth = labelText.size(withAttributes: attrs).width + 10
        // if you modify labelHeigh you will have to tweak baselineOffset in attrs
        let labelHeight = labelText.size(withAttributes: attrs).height + 4

        // place pill above the marker, centered along x
        var rectangle = CGRect(x: point.x, y: point.y, width: labelWidth, height: labelHeight)
        rectangle.origin.x -= rectangle.width / 2.0
        let spacing: CGFloat = 20
        rectangle.origin.y -= rectangle.height + spacing

        // rounded rect
        let clipPath = UIBezierPath(roundedRect: rectangle, cornerRadius: 6.0).cgPath
        context.addPath(clipPath)
        context.setFillColor(UIColor.white.cgColor)
        context.setStrokeColor(UIColor.black.cgColor)
        context.closePath()
        context.drawPath(using: .fillStroke)

        // add the text
        labelText.draw(with: rectangle, options: .usesLineFragmentOrigin, attributes: attrs, context: nil)
    }

    override func refreshContent(entry: ChartDataEntry, highlight: Highlight) {
        labelText = customString(entry.y)
    }

    private func customString(_ value: Double) -> String {
        let formattedString = PillMarker.formatter.string(from: TimeInterval(value))!
        // using this to convert the left axis values formatting, ie 2 min
        return "\(formattedString)"
    }
}

Then activate for your chart

let marker = PillMarker(color: .white, font: UIFont.boldSystemFont(ofSize: 14), textColor: .black)
chartView.marker = marker