overte/interface/resources/qml/controller/AnalogButton.qml

import QtQuick 2.1

Item {
    id: root
    property int size: 64
    width: size
    height: size
    property int controlId: 0
    property real value: 0
    property color color: 'black'

    function update() {
        value = controlId ? Controller.getValue(controlId) : 0;
        canvas.requestPaint();
    }

    Timer {
        interval: 50; running: true; repeat: true
        onTriggered: root.update();
    }

    Canvas {
       id: canvas
       anchors.fill: parent
       antialiasing: false

       onPaint: {
           var ctx = canvas.getContext('2d');
           ctx.save();
           ctx.beginPath();
           ctx.clearRect(0, 0, canvas.width, canvas.height);
           var fillHeight = root.value * canvas.height;

           ctx.fillStyle = 'red'
           ctx.fillRect(0, canvas.height - fillHeight, canvas.width, fillHeight);
           ctx.fill();
           ctx.restore()
       }
    }
}